There is a plethora of feedback here on this very forum on both (The search feature using LMT, or Troy will turn up lots of discussions)
But since we are here.,........
The Troy modular forend is a very worthy, popular, and highly recommended product. It is among the top 4 or so similar rail systems. Those would include KAC (Knights), Daniel Defense, and LaRue Tactical. The Troy often gets a nod due to excellent customer service, and their reputation gained as one of, if not these best BUIS available.
The LMT MRP is an awesome product, but is a specialty item in my opinion. It offers the user the ability to change barrels and set ups. This is not a requirement of mine, but many love it.
In addition to the barrel change, it is a continuous rail - uninterrupted its full length making it a good choice for Aimpoint and EOTech users who move their optic up or down the rail for optimum efficiency
Lastly, it is pricey. I think it runs about $1,250 making it prohibitive for many
Below are a couple of good links to further the background on these two items. Particularly the LMT MRP which has 40 pages of discussion - positive and not as positive.
Welcome to AR15.com, Mitch!!
The consummate LMT MRP thread Troy Rails, and many others. Has run for a couple years, so check the opening and later comments for up to date feedback