I always suggest "flat top" to new AR purchasers... Here at work, the two that listened are both VERY glad they did, and the one that didn't, well, he hasn't said he wishes he got a flat top, but he sure doesn't say "Man, I sure am glad I got this permanent carry handle" like the other two guys always say "man, I didn't think I wanted this to start with, but I sure am glad I got it now!"
It's a little cheaper to buy the flat-top, and a little more expensive to buy a detachable carry handle or BUIS.
I sold my Bushmaster carry handle as soon as I got it, and used the cash to try out lots of BUIS models. Funny thing is, the first one I bought was an ARMS #40. Then I bought a #38 EX, then a DPMS Detachable, and I just recently sold it and bought .......
An ARMS #40
FWIW, my first upper had an A1 on it, and after I found out about the flat-tops... I couldn't WAIT to get one.