We've stayed at M-Baileys several times, like it very much (especially when we got a good rate on Londontown). You can't beat the location for convenience -- you're in a neighborhood, not a tourist area, and just across the street from Gloucester Rd tube and a bus stop.

On our first visit, we went into the M-Gloucester by mistake. It seems to be a typical modern hotel: we preferred the more traditional Baileys. Folks who work at Baileys are very pleasant, the rooms are not cookie-cutter same but clean and well appointed. Haven't eaten in the restaurant, but they have a bar that is very pleasant for a nightcap.

If price is the same for both, I think you won't go wrong whichever you choose. Of course, depending on your needs and/or expectations, YMMV...