Side all the way. Lot more tempting. If a cleavage was an ass crack, the side tit would be a cute bikini bottom ass. You know what I'm talking about, fellas.
definitely side boob. cleavage works well for me when the girl has slightly small to a bit above average boobs. i don't like it when the boobs are huge. it looks unnatural and it's usually some fat girl who is trying to flaunt the one thing she has.