LaRue or DD, you really cant go wrong with either, although I am partial to LaRue.
I am not a fan of the LMT MRP due to its using a proprietary barrel, and personally I think quick change barrel availability is over-rated. Reason being, is you will still need to re-zero your sights (optic & iron) after barrel changes. So why not have a complete spare upper assembly.
Now, the monolithic railed upper does have its benefits for mounting various optics, IR's and NV. But I dont have a need for such devices.
What would be really neat is if, say the DD locking collar for the lite rail, was an integral part of the upper reciever. This way you could put any length DD-Lite rail on it that you desire.