I have 2 Micaiahs, Neutral and +SPD -ATK. What type is better for her?
+SPD with 4 Dragon Flower can reach 32 SPD plus with her Tome and if enemy is debuffed by her B exclusive passive she gains 12 SPD, 44 SPD total without buffs (or Darting Blow 3 Seal). And if she can’t debuff speedy enemies, other ally with Chill SPD (Naga for example) can support her very well…
But… I’m not sure if +SPD is a good idea an Neutral is better.
+Spd, because you already explained why.
Forgot that her weapon adds debuff of enemies to her own stats. Yeah +spd is better between the two.
I know +ATK is the best type, but probably +SPD is the second best choice or between +Res. Well, thanks for your tour opinion
Welcome!.
Since her weapon has double effectiveness (thus can reach high damage on cavalry and armors) try out +spd!, the stat can be salvaged if invested, the build you propose looks good for her! (also rare but there are a very few speedy armors, L!Tiki is an example!)
Welcome to Gamepress. 
I had the same predicament as you and actually chose Neutral IVs because she gets more points in her Atk and Res that way. Unless you’re going for “gotta go fast” Micaiah, I feel the neutral IV one offers more utility with a merge.
Just to chime in, there’s an argument to be made for neutral since it gives you 1 more res.
Neutral gives +hp, +res, +atk instead of +3 spd. I’d probably go with neutral personally. Having higher hp and res is just more important for support unit stats. I’d give a mage like Nino +spd, but that’s not what Micaiah is.
I don’t think there’s any question, +res is the best iv bar none. Res improves her ability to tank mages, it improves her b-skill, and it also gives more damage when her special goes off.
Neutral is good too with merge but i probably +SPD with Darting Blow 3 Seal (or SPD Buff) is better to defeat speedy units without DC, plus with Divine Fang can destroy non armored dragons like Nah, Fallen Corrin (F) etc… and Naga can run with Chill SPD, Naga is the perfect partner for her. I tried B!Micaiah +SPD with her default Build (Except seal) and she has better survivability and win rates than merged Neutral. In enemy phase no differences.