Selecting a rail system depends on many factors. You should keep in mind the purpose of the rifle, the optics you will be using and the other accessories you will be mounting such a lights, lasers, etc. Last but not least, personal preference will play a big part, always get the system that works best for you!
As I mentioned on my project post,[url=http://www.ar15.com/forums/topic.html?b=3&f=12&t=162739]M4 Project Notes[/url], I originally planned on getting a Knight's M4 RAS(Rail Adapter System) but as I did some more looking around I found that there were other rail systems that had better features while not costing much more.
Some of the problems that faced the RAS system were the inability to securely mount optics, the heat shields protected your hands but actually kept the heat in the hand guards rather than dissipating it, the RAS did not free float the barrel causing some issues as more accessories were mounted.
I decided on the arms SIR 50 system because of the following advantages it offered.
1. Completely rigid full length rail for installing optics, lasers, etc.
2. Completely free floats the barrel.
3. Excellent cooling, bested only by older larger model SIR 45.
4. Rails of different sizes can be added or removed as needed in the 3, 6 and 9'clock positions.
5. Bottom half of hand guard is polymer and can be removed to access the barrel for cleaning.
6. Bi-level feature allows for co-witnessed forward mounting of EOTech optics.
7. Integrates perfectly with the ARMS 40 BUIS.
8. Easy to install requires no special tools or rifle disassembly other than removing the hand guards.
The other deciding factor was price. The Knight's M4 RAS was selling at $285-$300 and I was able to find a dealer selling the SIR 50 for $355. I felt the small extra amount was well worth all the advantages.
Note the older SIR models, the 45c(Commercial/Civilian) and 45m(Military) were a bit bulkier and the 45m required removal of the delta ring for installation. Ironically the preference of the military was the C models which allowed you leave the delta ring and easily switch another configuration if needed.
Knight's has a Free Float RAS which is very nice system but it requires removal of the front sight and replacing the barrel nut for installation. Other the more difficult installation I felt it would also take away from the versatility to switch to another system if needed.
Knight's also has a newer rail system the RASII which has many of the features of the SIR 50. It's a bit less around $330 and is also a very good system. I still felt the SIR was an overall better system especially for my needs, still the RASII would be my very close second choice.
So far I'm very happy with the SIR 50! It has perfromed as advertised and the way it mounts makes it feel totally part of of the upper receiver. If I do my part my carbine can easily do 3/4 MOA and the SIR keeps everything nice and cool during extensive rapid fire sessions.
Here's some some pics of the RAS, FFRAS and RASII.
M4 RAS
[img]http://quarterbore.com/images/kacm4ras.jpg[/img]
FF RAS
[img]http://quarterbore.com/images/kac_ff_ras_family.jpg[/img]
RASII
[img]http://quarterbore.com/images/kacrasiij.jpg[/img]
Here's some of the other SIR systems.
[img]http://www.armsmounts.com/images/sirs.jpg[/img]