I'm on the hunt for a place as well that can accomodate about 50 people for the rehearsal dinner. I'd prefer Mexican food as well.
I was at Mi Casa in February for a bachelorette party with 15 girls. THe service was pretty good and the food was OK. I wouldn't say it's "authentic" Mexican food. The ambiance is cheesey but fun. It has a very festive vibe. THe only reason I am not considering Mi Casa as an option for my party is I heard it's $100 per person and I really just don't think the food and the margs (made with the low-level tequila) are worth it.
I just emailed La Fonda, which supposedly has great Mexican food. I'll let you know about their pricing and any other options I come across.