I've taken a look at pictures (on google) and videos (on youtube) of Mila Kunis and I still have to say Kristen Bell would be taken from behind before Mila ever would.
Mila isn't 1% as hot as Kristen. Kristen has a better face and body and personality and seems like so much more fun to be with. I LOOOOOOOOOOOOOOOVE Kristen. Mila - not that hot - especially w/o makeup. YIKES
After watching interviews with Kristen Bell, she strikes me as potentially batshit crazy. So, what I'm saying is... Threesome with both of them. My wife can hold the camera or jump in.