Well, take into consideration size, chamber room, how many chambers, any other filtration processes, etc. Go with the bong that best suits you though... there's no need to go off spending 300 bucks on the best bong you can find, because if it's broke, it'll break along with your heart.

If it's just a PIPE... then it's kinda just a "hey it looks good, I'll take it!".

Goodluck searching man, it's always fun searching for pipes and bongs.