This sort of goes against your whole topic, but I prefer a Knight's RAS railed handguard with a #38 EX-MOD typing it all together. The #38 EX-MOD rail secures the RAS to the flattop and makes it just as rigid as the ARMS SIR series. Plus if you ever put a "can" on the end, it helps support the barrel so it doesn't droop from the weight under heavy fire. The #38 EX-MOD's function is actually opposite of the SIR system's free-floating ability.
To me its part of the whole modular thing. I can take the top rail off and mount stuff directly on the flattop, or put the rail on and mount a scope on top of it, without the need for a spacer.
Plus the ARMS #38 EX-MOD rail has a built in ARMS #40 flip up sight. Something the ARMS #45 lacks. If you factor in the cost of a back up sight to the SIR price, they both come out to about $500 a piece. To each his own....