kmitzvah,
Any company you meet with should be able to provide you with a list of families to call. Make sure the families are from recent parties, and not a party done a six months ago.
Your choice should come down to who you're more comfortable with. If you want a high-energy, over the top celebration - you'd be more comfortable with MC X, whereas if you wanted something exciting and free flowing - you'd be comfortable with MC Y, etc. Make sure to find out how each DJ/MC runs a show - even check them out at a party if time permits. Your DJ is going to make or break your party - people will leave raving about the great time they had if the DJ is amazing, conversely they'll say a quick thanks and then quickly head out if the party isn't up to par...possibly even leaving early.
One DJ might be better suited for a party focusing heavily on games, one might be better for a party focused more on dancing, etc. Are you having adults and children at your party - make sure your DJ is able to cater to both crowds.
I'm happy to discuss with you in more detail - and to get you some contact info for our recent clients -

. Hope to speak with you soon.
Geoff Sockol
Electric Entertainment