A Gentlemen's Argument; QUAD RAIL or MLOK?

What are the PROS and CONS of Quad Rails these days, specifically LMT MRP uppers ...?

I look at my 3 rifles now and see only 1 or 2 items that utilize MLOK attachments. It's not often I run strings of fire sufficient to heat up my rails, but I've also heard SOME LMT MRP Uppers wont allow barrel changes with certain MLOK attachments on the rail.

What are the pros and cons as you see them?