One has detachable rails on the bottom the other doesn't.  Why is S&W making both of these models when they are essentially the same.  I guess the M&P gets the M&P treatment (charge holes, etc).

Am I missing something?

Anybody bought either?  I would interested in how you came to the decision on which one.

I have the R8 and it is by far the best handgun/revolver I have ever owned.  I can hit clays at 25-40 yards easily, which says a lot because I'm not known for my accuracy.  I can even do it in DA.  
I put a TLR-1 on it and it complements it well and hasn't failed yet and so far I have put 400-500 rounds through it, all .357 loads.  The recoil is very manageable and I can shoot it for 2-3 hours and not feel worn out.  My other revolver (M&P 340) with .357 loads isn't as nice, 10 rounds and I'm pretty much done.  
Trigger pull is lighter and smoother than my 340.  I had an LCR and the trigger on it was better, but it was DA only.  It is front heavy but I think the extra weight out front with the light helps.  
It's a unique gun and well worth it if you're going to invest in something like it.  You'll have a hard time finding a holster out there that accepts a light mounted as well, which is why I'm going to do my own.
Hope this helps you a little.
Ryan

Thanks for the info.  

Still curious why both models exist.

I think the R-8 was geared primarily towards the Police market, the idea was that an entry team shield man would have a gun that would not malfunction due to a slide on an auto hitting the shield. I think S&W(and this is just a guess) figured that a civillain would like a rail but not attached all the time hence the removable bottom rail. I have a TRR8 and it is one of my favorite handguns as well, I even carry it CCW once in awhile. I lucked out finding my barely used one last summer and as much as I like it I would have had a hard time coughing up the retail price.


Dan

Thanks for the info.  I like your hypothesis as yes I heard/read the same thing.  Geared toward the shield man as short armed guy might cause a FTF by boucing the slide off the shield.

I've wondered about this myself.  I've also been curious as to why those chose the five-inch length over a four-inch.

Cause 5" is Perfect! one of my friends had a pre-model 27 with a 5" inch barrel and that thing was Sweet.  I would guess that the 5" lets them get just enough rail length to make it useful. My TRR8 is one of my favorite guns.