Mostly upper body for me. You're into whatever you're into, but here are a few things about this for me:
1.) With upper body, the festish aspect is more covert.
If you're on a date, or flirting with someone, you can give her a quick index finger poke in the side, without her necessarily thinking "this guy has a fetish." This can present a whole big explanation or admission that you may not be ready to give anyone generally, or give that person specifically. It provides a lower bar of a gateway for engaging with many normies, who think of side or belly tickling as mainstream flirting -- but who would think of a "please take off your shoes" request as pervy or weird.
2.) The upper body usually doesn't require removal of armor.
Yes, there are times by the pool or at the beach, etc., where people don't wear shoes, but generally the midriff is far more often accessible without requiring prep work. So if you've already been physical with someone, the surprise attack factor is different.
3.) Midriffs don't usually have dirtiness or smelliness problems.
Self-explanatory.
Again, people don't choose what turns them on. But presuming you like both, I see these practical advantages to upper body fun.