For prestige they're pretty even. I would say that it doesn't matter whether you go with Mercer or TW, but what does matter is what you do when you're there.
One thing to look at is the list of top recruiters at the schools you plan to apply to. In my case, I'm applying to Columbia and Towers Perrin was a "top recruiter" pre-merger, whereas Mercer isn't on that list. (I'm ex-TP.)
I would suggest that if you're trying to decide between the two you should go with whichever one feels like a better fit when you meet people who work there. You're going to do the best work when you're happiest, and that's going to help you most when it comes time to do your MBA.
What practice area are you hoping to join?
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