Go with the 28, it's the largest you can get while having the thermoeletric system.
Also, I wouldn't worry too much about fans. I calculated it would take a little under 1.5lbs of heartfelt beads for a 28 count cooler, so I got 1.5 lbs. I'll put a half a pound at the bottom, middle, and top of the cooler and I'm sure that will be plenty of stability without needing fans. Other people might disagree, but that's just my
