Mewtwo

2251. The individual 1 attack iv won't differ much. At 14 attack at level 40 it still has much more attack than an alakazam of the same level with 15 attack ivs.

2251 mewtwo is a monster even at 10/10/10

Between the two, absolutely the 2251. Mewtwo's base attack is leagues ahead of every other Mon, so losing out on 1pt of ATK IVs won't make any noticeable difference.

Both.. why else would you power up over a second mewtwo?! I had a 2210 that’s maxed out. Hopefullly my second will be better.

I would power up both. But actually I prefer the ATK 15 one.