Two factors that will help you make your decision. How much boost you want to run, and when you want it to spool. The 660 in theory would make the same power on lower boost compared to the 550. I'm always a fan of slightly oversizing the turbo, just so you're not pushing the thing to the limit, but the 550 seems to be in that ballpark, where the 660 would be a bit overkill for 400 hp. As far as the A/R's, I looked into it a bit, and comparing it to the 2871R, I see that they run larger trims for the 550/660, but that seems like a really small A/R compared to the 2871's smaller .64.
If it were me, I'd be leaning more toward the 2871, it's a tried and true size that lots of people use to hit that power level, and with the smaller A/R I hear that the spool up isn't too bad, but with the larger one it can feel a bit laggy.
This may of helped, it may of not, but good luck on your decision either way
