A fairly obvious one this, but if you’re into long practise sessions, you’ll need a bench that’s comfortable and supportive with at least a minimal amount of padding - if what you’re sat on is not comfortable, you’ll not want to be sat at the piano for any length of time, so a degree of comfort is important when choosing your piano bench.
Most piano benches are pretty stable, too, and that’s a good thing - the last thing you want is for your seat to collapse while you’re rocking out at the keys. The traditional solid design with a leg at each corner will generally be a bit more stable than a folding x-type stool, but since these are much more portable, they’re an attractive prospect for gigging musicians.