My main driver is price. I've been willing to put up with a format's weaknesses if it meant deep discounts. Can't use iOS nor Android since those are digital only, but otherwise, they get their fair share of sales.
For PC, I've gotten many Steam games through their sales, and I got enough games via Humble Bundle that the price per game is easily $1. It's higher if you cut out the "bad ones" or ones won't have time to get to, but that's still a stellar proposition.
For consoles and handhelds... I end up preferring physical for that reason b/c that's how it's been here. I wait 5 years later, and Super Mario Galaxy 2 is now only $20 vs. $60. I picked up a bunch of games on SD card for Tapwave Zodiac (Palm OS handheld that focused on gaming) for $5 a pop. I hear 3DS games also go on sale. For the latter 2, buying digital versions of games remain at full price. AFAIK, it's because they need to move physical inventory, whereas digital games don't have that same issue to be that costly (that's right... a while back, but long after the Tapwave Zodiac got discontinued, their games were STILL full price via digital purchase).
TL;DR - I go by price, but after that factor, I'd go with cartridge as my top pick. But they have to be small. Doesn't have to be as small as the Switch, but definitely not NES or SNES. Not much reason they needed to be that big (beyond marketing)