[militarymoron]

i prefer the KAC RIS to the SIR myself. i have the SIR #50 (slimline, bi-level); here's my personal and non-professional .02 about the SIR.

likes:
1. easiest free-floating rail to install (at the time i bought it). both the civvie and military version are relatively simple to install.
2. cooling (like reaper mentioned), but i use a vert grip so that doesn't really affect me

dislikes:
1. raises the flat top up. cheek weld suffers (slightly) if using an ACOG. eotech cannot co-witness (unless mounted on a bi-level on the front).
2. you can only use an ARMS BUIS.
3. adding side or bottom rails requires tools (a coin or screwdriver) and you have to find your rail.
4. doesn't come with full length side or bottom rails (you might not need them).
5. plastic lower handguard has some sharp edges - not as comfy as the standard M4 handguards or a RIS with rail covers
6. more bulky than the KAC with rails exposed
7. need a full length rail (#4, not supplied) on the bottom when using a vert grip - the short ones don't provide enough support with one screw and tab
8. none of the rails on the SIR are mil-std 1913 compliant. not a prob with most accessories, but won't fit a TD vertical grip which IS 1913 compliant

i just think the KAC does it in a more compact package, but it's just personal preference. a FF RAS would be one of my top choices (i have the KAC FF MRE version)
but i like the LMT MRP better than either the SIR or the RIS/RAS.
cheers,
MM