I would go with full-skip for 28" or 32"; for 24" it's a toss-up; and for 20" the choice is clearly full-comp. I've run full-comp on that class of saw (7900 Dolmar) with a 28" bar and much prefer full-skip if the bar is going to be buried. Full-comp slows things down more that I would prefer. As for the specific chain used, I will assume that full-chisel, round ground, so the usual rules apply - Oregon dulls fastest but cleans up easiest, Stihl is the opposite. WoodsmanPro is the best bet, I feel, if you're into ordering your chain.
As for the 28" versus 32", I think that 28" is the practical limit in hardwood for this size saw if time is an issue. It will seem slow enough with the 28" setup and super slow with the 32" after running a 20" bar.
