Okey dokey.... I'd up the ram to as much as you can afford to have.
Reason being, 27inch has 2 free slots and you can access the ram and put more in later. The 21.5inch is sealed, there's no user access and you can't add more ram later and with 8gig and the burgeoning load the OS puts on ram requirements you could well run short very quickly.
It's a shame Apple took away the user accessible door in the 21.5inch as their ram prices are extortionate. In all other ways your proposed iMac is cool.