Run the room choices on oyster.com to see what you like best. I would personally rather stay at Mirage, BUT their (Mirage) basic rooms don't have floor-to-ceiling windows. Plus, if you get the basic, basic room, it's a pretty, but tiny bathroom...with a shower curtain. A few dollars more gets you the tower room, with a separate tub and walk-in shower. (I don't need a big room, but a shower curtain bugs me.)
The reason PERSONALLY would pick Mirage is that I love the beautiful atrium of Mirage and the gorgeous fish tank behind check-in, but these might not matter someone else.
Also, since you're already at Cosmo, you can hit surrounding properties at Cosmo, and then have a "whole new trip" once you hit Mirage, as you'll be on a totally different part of the strip.
Either should be solid, I just would check out photos online first, to see what you prefer.