I think the hotel's main (Marriott) competition is the Sheraton Maria Isabel and I stayed at both this month. Unfortunately I have not stayed at the Marriott Reforma. Rates are sort of similar, sometimes one is more expensive than the other.

Pros for LM:
+ I got a Superior room which is a proper (non-junior) suite with separated living room. Honestly it looks pretty similar to ElevatorEnthusiast's room above since all rooms are suites. There's a lot of space and I didn't really notice wear and tear, though I'm not picky. At Sheraton I got a "Business Deluxe Club" which is a normal room with club access.
+ The pool is much nicer. The Sheraton pool, while on the rooftop, is tiny and too small to actually swim in. It's also heated and warm

Pros for Sheraton:
+ The club lounge has pretty good food and the breakfast is much better than the LM breakfast. Sheraton had eggs and juices made to order while LM did not when I was there. Sheraton club lounge also has evening food/beverages. You can also print documents for free on the computer
+ The gym is much nicer. Lots of machines. The LM gym is pretty limited and small
+ Location is slightly better. I take Uber everywhere in CDMX that's not super walkable. Sheraton is closer to Chapultepec and Roma (as well as the flagship St Regis and Ritz Carlton) while Le Meridien is closer to the Palacio de Bellas Artes etc. Not a huge deal if you use Uber
+ Service in the lounge is good. I didn't really interact with LM staff much but it was better than LM's breakfast staff.

If you want space go for LM but if Sheraton is cheaper I'd probably go back there