"Bip" is our word for hot or cute.  For instance, we think Howard Dean is bip.  It is also an exclamation; for example, if you met Jude Law, you might say "Bip!"

 

So, essentially, "Who's bipper?" means "Who's hotter?"

 

Note: If you can't choose which boy is bipper, choose "neither" or "both"-- "neither" if you can't choose because you think they're both ugly, or "both" if you can't choose because they're both too cute!