Hello everyone, I wanted to get some opinions from other people here. Advice, thoughts, input, whatever.

I've been having a conundrum for some time now. I know that most NPC stat blocks are given their HP according to their size, meaning most humanoids end up using d8's. But I'm having some issues with some decisions. Primarily with spellcasters, because you can see the issues with their HP more. Most of them have more hit dice than they have levels in spellcaster classes, I understand this and the fact they use d8's is to keep enemies from being overly squishy against your players, it annoys me a tad for some reason I can't explain, but I understand it. 

My main issue is that I'm planning on having another group on NPCs to be like a rival adventuring party to my players, keeping the same level and everything. So I figure they easiest thing to do would be to build them actual character sheets and give them levels, HP, Items and the like as the players would. So my issue is, should they all use the hit dice of their classes, or just give them extra hit dice to make them more like an actual NPC?

I realize I'm overthinking it and all, but I just want other people's input on having NPC's leveled like characters with classes.

If you build them like PCs, expect a short , very swingy fight; PCs have very high damage output relative to their hit points.

Also keep in mind that if you intend to have these groups "face off" it will become a very dice-ruled combat.  If the rolls swing one way or the other for a round or 2, one party is going to get wiped rather quickly.  I believe the NPC blocks would be best and by all means, use the PC sheet as reference for HP or AC or some such.  Building an actual PC tends to give a much higher challenge than one might expect at times.

The other issue with using PC stat blocks for NPCs is that PC stat blocks are a lot more complicated than NPC stat blocks, which can be a headache for the DM.

Well. remember you control the Second parties stats.  if you are using one system for the party cut the ability of the NPCs to what you want them to be.
If you have a DM subsciption you have room for as many characters as you want.

With NPC’s I often start by building a full sheet to tweak it and get a chance to think through the options then build it out in a stat block.

I pretty much always max out the HP for anything my players have to fight. The party has some pretty good DPS, so monsters that are supposedly challenges for them usually end up destroyed in two rounds. It's always easier to back off in the middle of combat than to ramp up if you find you didn't balance the encounter well. 

The other issue with using PC stat blocks for NPCs is that PC stat blocks are a lot more complicated than NPC stat blocks, which can be a headache for the DM.

This, right here, is why I have never, and will never, used a PC stat block for an NPC. Nor do I give NPCs the same # of different abilities that each class of their type gets.  

Remember that, unlike a PC, an NPC, unless it is a "party NPC" who is going to stick with the party long-term, will only be in the action for a short while. If it's combat, this NPC will have a life-span of rounds that can probably be counted on the fingers of one hand. What is the point of giving that NPC 27 different "class abilities" for whatever class they have, when they're only going to maybe be able to use 3 or 4 of them before dying -- or if an ally, before the combat ends? 

Consequently, I use monster stat blocks. I make up my own stat block. In terms of the dice concern, screw what MM says, if it's a class that gets d10s, I give 'em d10s. And then I look up the class abilities for the approximate "level" of the NPC, and use not all of them, but pick out a handful (no more than 3 or 4) that I think are most likely to be useful or possibly come up, and I give them those. I carefully screen abilities for things like having to track some other freaking kind of resource, like superiority dice, and most of the time those just get weeded out because, as DM tracking multiple monsters and NPCs at once, I'm never going to remember to do that anyway, and I don't need the extra headache. Because it is just too much book-keeping on top of everything else. 

Now, keep in mind that in a sense this means that the NPC could potentially be "less powerful" than a PC of the same "level" (since the PC can do many more things), but if you do it right (and it does take some practice), the abilities you cut out will be ones that aren't going to be used in the encounter 99% of the time anyway.

Use the classes and sub-classes as a theme for an NPC, and then pare away all the meaningless stuff, while adding anything you think will enhance the NPC and the encounter. That is what I think Bio is doing.

That's what I do.

Can't take credit. Coleville does it and advises it. Fortunately I saw his advice before I started doing much DMing.

I would always recommend using a "monster" statblock for NPCs for simplicity, but also for balancing.  PC levels and CR don't really align very well at all (CR is generally stronger than the same level by a fair amount).  If this is a combat encounter, you can cut out all of the class abilities and non-proficient skills that are irrelevant in combat, and others can be greatly simplified in the monster statblock.  

I typically don't reduce spells too much;  2-3 per spell level is pretty typical for Monsters, so it winds up being somewhat comparable to PCs.

Rather than reply on each response individually, I want to thank you all for your input. Sorry if it doesn't notify you, but I am thankful for all of it. In essence, take it easy on myself and just make em "monsters" XD Thanks again everyone, hope you all have a great day.