Which feat to choose for my arcane trickster melee charging rogue?

Hi, friends!

I have to choose a new feat for my Arcane Trickster Rogue, and I can choose a feat from either 5e or 5.5e. I desperately need help choosing. 

The feats she already has: 

• Charger (+10 Speed when Dashing. Charge Attack: Run 10 ft in straight line and add 1d8 extra damage to Melee attack)
• Fey-Touched (New spells: Misty Step and Hunter's Mark)
• Piercer (Reroll piercing damage roll once per turn. Add one piercing dam die when critting.)

Some background: Her signature fighting style is charging ahead in combat for extra damage with her rapier casting Green-Flame Blade for extra fire damage and disengage to get out of reach. She does not have a high health pool, at 67hp. She's now level 12. She's got 20 in dex and 19 in Intelligence, which means that preferably I would like to get an increase in int. 

The feats I am considering:

• Shadow Touched (5.5e) Intelligence: +1. New spells; False Life and Invisibility. This will help me as a front fight charger, in terms of hp and an added elemental of magical surprise.
• Elemental Adept (5.5e) Intelligence: +1. Ignore Fire dam resistances and treat every fire dam roll of 1 as a 2. Will help my spellcasting and my Green Flame Blade attacks (as well as Firebolt) which I use all the time as part of my normal melee attacks.
• Mobile (5e) Speed increase 10 ft. Difficult terrain cost no extra speed while dashing. No provoking opportunity attacks when making a melee attack. This helps my playstyle immensely, by being able to fly in - attack - and fly out again without having to use my bonus action on disengage! But I wont get an increase in Int.

Thoughts anyone?

My initial thought is that Mobile would be a really good feat for this character, and the opportunity attack immunity alone is going to be worth way more than a +1 to Intelligence, especially when your Intelligence is already very high for an Arcane Trickster at that level. It will mean that a lot more of the time, you'll have a bonus action free to do stuff other than Disengage.

If you find yourself very often casting spells that actually use the spellcasting ability, that extra +1 to Intelligence might be worth it. But most Arcane Tricksters don't ever push their Intelligence that high, and prefer stick to spells that don't use it.