The rifle is an M4 configuration.  When I finally get an optic, it will be an Eotech.  What I want is a FF RAS with which I can cowitness the Eotech and a BUIS.  I've seen pictures of the ARMS S.I.R., the Knight RAS II, and the M4 FF RAS ...was curious about opinions.

Also, in all the pics of the M4 FF RAS, there's been an flip up front site.  Is the standard front site always removed/replaced with this RAS system?

I have a free float RAS, and I use the standard front sight.  It is always the case the front sight are replace, the norm is to keep the standard front sight. Especially if you depend on your BUIS to support your red dot in case of failure, it would double your time to flip the front and the rear sight.  keep your front sight, and you will be better off.  I do suggest transition to your secondery weapon instead of fliping sights.

With FF RAS, if you have a post ban gun, it will take some serious gun smithing to remove you muzzle brake so you can remove your front sight post, so you can remove the barrel to install the FF RAS.  this is why RAS2 and SIR is so popular.  something to consider.

All the system you listed are excellent choice, but in my opinion, SIR and FF RAS stand above all.

Wes - the platform will be an M4gery.  My only requirements are that it's a FF and I can co-witness with an Eotech.  Thx.

You say "will be" so I am assuming this rifle is not yet assembled.  If that is the case do what I did and just buy the FF RAS since it will cost the least and work best of the products you mentioned.  You can purchase a 14.5" barrel with no device attached and after installation of the barrel, FF RAS and reinstallation of the front sight put on your muzzle devise without incurring a monetary penalty for having to remove a permanently attached muzzle devise.

Our Folding Front Sight for the end of a RAS is P/N: 99051, and it is an option.
It can be used on the M4 RAS, RIS, or RAS II, as well as the Free-Floating RAS for which it was originally intended.  Note that on the shorter RAS's, its front sight post may need to be re-adjusted "higher".  Usually this is not a problem, as it lays down most of the time.
I note that its becoming more popular to cut off the standing front sight post as the reliability of selective primary sights such as the Aimpoint COMP (M68) and M2 are so much better than just a few years ago.  This gets the image of the original sight tower "out of your face", and maintains the best gas seal your barrel will ever see, as its original gas block remains in place.
You can do this pretty well with the front sight tower factory assembled to you barrel.  But the nicest looking ones I've seen (and done) require the gas block to be removed so you can support its flat rear surface on a belt sander table.  This lets you put a nice radius accross the top where the sight tower used to be.
Then when you re-assemble and are usinfg a RAS II ofr a FF, you have the option of not using the original forend cap, which will let more air circulate within the void of the handguard.
ColdBlue sends...

DevL ...you're correct.  The rifle hasn't been assembled yet.  The upper is being obtained "piecemeal".  Right now all I have is the barrel (with no FS), with an M4 receiver to follow soon.

Coldblue ...a lot of info there, thanks.  I want to leave the orginal sight intact, as my plan is to do this project on my own.  Anything that would require a gunsmith I'd prefer not to do.

If your barrel does not yet have a FS (front sight) and you don't want a "choice" that requires a gunsmith, I assume you are going with an "issue" standing front sight tower/gas block/bayonet lug assembly?  Since these are usually "factory installed" on the barrel with tappered pins, and consequently not interchangeable between barrels, I don't understand the no gunsmith part of your plan?
ColdBlue perplexed...
Also note that if the FS is not yet on the barrel, then you have "all" the RAS choices available to you.

Coldblue ...mix up with abbreviations.  The barrel doesn't have a flash suppressor, but does have the standard tower front sight.