I'm making an half elf rogue, and I'm thinking of multiclassing into a draconic sorcerer for the Ac and extra health, along with a few spells. (I have 16 charisma) for my four cantrips should I take true strike, just for the advantage, or should I get booming blade?

True Strike is on the short list for worst spell in the game. Take booming blade, or pretty much anything else.

You wont get any extra health the extra hit point you get by draconic resiliance is cancelled out by only getting a d6 hit dice rather than d8.  In fact you will be worse off health wise because you will have slightly less hit dice recovery available during  short rests.

Assuming you have studded leather armor you will gain +1 to AC which isn't enough to justify multiclassing remember for each level you multiclass you will be half a dice behnd on your sneak attack damage and getting all the sweet features like uncanny dodge, expertise and evasion a level later.  Multiclass for role-play reasons or possibly to become a caster not for the AC.

If you do multiclass, I am not a big fan of true strike it takes an action which means you can't get an attack in that turn and 2 attacks are better than 1 (a hit and a miss or two misses has the same effect but two hits and you are better off with two attacks).    True strike does have some uses for example if you have set up an ambush you can hacve advantage on your first attack by castng true strike first or if it is difficult to get sneak attack every turn but can if you are using true strike, but I feel it is to rare to be worth a cantrip known, I would go booming blade.

True Strike is almost always completely and utterly awful.  The cases in which it's not terrible - note, those words were "not terrible", as opposed to "good" - are rare and esoteric, even for rogues.  I don't know as there's a single cantrip on the entire sorcerer list I wouldn't take before I took True Strike.

This particular spell took it deeply in the pants during the switch from earlier editions to 5e.  Avoid at all costs.

No. True strike is mathematically worse than doing ANYTHING else with your action under 99% of all circumstances. There is no other way to put it. That spell should have to go to the corner and think about it’s mistakes.

True Strike it horrible. If you want to attack, just do it twice. The spell takes an action, only lasts a round, and is concentration. It is, IMO, the worst spell in the whole game. 

Quote from Yurei1453 >>

True Strike is almost always completely and utterly awful.  The cases in which it's not terrible - note, those words were "not terrible", as opposed to "good" - are rare and esoteric, even for rogues.  I don't know as there's a single cantrip on the entire sorcerer list I wouldn't take before I took True Strike.

This particular spell took it deeply in the pants during the switch from earlier editions to 5e.  Avoid at all costs.

For theory's sake the only situation I can think of in which True Strike is upgraded from terrible to simply meh is if you've made an Elven Accuracy crit fisher, you have absolutely no other means to get Advantage in the moment, and you have one turn before combat starts to prep.

I made this character as a duelist thinking that he'd be challenging people to honor duels by pointing at them and casting True Strike. It's never happened even once. When my DM allowed Spell Versatility from the class variants UA, I switched it out immediately.

If I were to redesign it, it would have it's duration changed to one hour, no concentration, and specifically state that it is the only spell whose casting does not break invisibility.    Then it would be worth taking and you can reasonably expect to use it at least once a battle.

Quote from Mog_Dracov >>

If I were to redesign it, it would have it's duration changed to one hour, no concentration, and specifically state that it is the only spell whose casting does not break invisibility.    Then it would be worth taking and you can reasonably expect to use it at least once a battle.

I always thought it would be more useable if it were a bonus action.

Quote from Ophidimancer >>

Quote from Mog_Dracov >>

If I were to redesign it, it would have it's duration changed to one hour, no concentration, and specifically state that it is the only spell whose casting does not break invisibility.    Then it would be worth taking and you can reasonably expect to use it at least once a battle.

I always thought it would be more useable if it were a bonus action.

 That would be too abusable. It would be an autocast almost every turn unless you nerfed it a bit. Advantage is just too strong. This is the best fix I have come up with, it’s okay, but not great. I would love to improve upon it if anybody has any suggestions.

Edit: It helps if I include the link. 😒

https://www.dndbeyond.com/spells/421057-strike-true

True Strike is very situational. If you want to play an assassin, it might help you score on those moments where you have to silently take out guards and whatnot before they can react. Outside of ambushes, though, it's not really worth it.

It's not worth it for ambushes either. In an ambush, you attack a creature that is not aware of your presence. Unless you are morally opposed to using a ranged weapon, you will already have advantage on an attack when you attack unseen.

I'm sure theoretically there's a use for True Strike that makes it worth taking for some people.  Theoretically.

In practice it's trash.  Don't take it.

Quote from scatterbraind >>

It's not worth it for ambushes either. In an ambush, you attack a creature that is not aware of your presence. Unless you are morally opposed to using a ranged weapon, you will already have advantage on an attack when you attack unseen.

 Outside of those one to three times per campaign that you face a creature that's immune to being surprised.

It might theoretically be worth it were it changed to taking a bonus action to cast and having it apply your spellcasting ability modifier to your next attack roll rather than granting advantage.

The only time I could ever see it as "useful" is if you have a high level spell that requires an attack roll and you really want to avoid wasting the spell slot.

Another I can think of that is,  if you have a Bonus action you really want to get off and can't attack them for that round. There is a slim chance it could be useful.

Eg. Warlock casts Hex, but can't attack with eldritch blast (for some reason???) and was not in range for melee.

But the general consensus is just don't.

Since True Strike requires you to see the target and them to be within 30 feet, there's basically no situation where you could use it on an enemy you couldn't hit with EB or some other offensive cantrip.

Quote from 6thLyranGuard >>

Since True Strike requires you to see the target and them to be within 30 feet, there's basically no situation where you could use it on an enemy you couldn't hit with EB or some other offensive cantrip.

 This^ Heck, Thorn Whip could hit that target.

I think a bonus-action instant-advantage would work if it was a leveled spell. It'd be great for multiclasses and half-casters, and generally useful for dedicated casters. Probably most useful for Warlocks, since they have the beefiest cantrip to pair it with.

Thanks! Maybe I'll take Find Familiar , for the help action,and Sleep.