play with the one you feel most comfortable with. if it feels to heavy, demo the MP-99. If that feels better, you can switch. IMO, yes, it is worth it. of course, i find the AT-700 unbearably head heavy.
If they're both in the same condition, like new or already messed up, do it. But then again, if you're already used to your AT700, it's your choice. I like the MP99 better but that's my own opinion.