A vote for face down also; renders the 'lee much more defenseless IMO, though creating a something of a challenge to get my fingers on her belly and lower abdomen, unless I can seize the opportunity of her struggles and laughing heaves to get my hands under her lightning quick.
I'm fortunate to be over 6 feet tall and "well constructed" of upper body (probably more fatty tissue than muscle that I'd care to acknowledge) and my late wife was considerably shorter than me, so I was often able to pin her down with my fulcrum near her lower back, from which position I could easily pivot back and forth from attacking her sides and pits down to her feet, and along the way having a go at her exposed super-ticklish underlegs and butt. My jungle-grade long arms aided my versatile reach too.