We have a 262XP in the line-up, excellent saw at all levels. I would NOT want to use a 24", and wouldn't even recomend a 20" bar for it, at least for the type of cutting that we do. Doesn't mean it wouldn't pull either of them, but we cut a lot of tops, and speed is everything, at least if you want to get anything done!
The 262XP is however, very happy with an 18" bar 3/8" full chisel. I can say for certain power ratings are somewhat arbitrary, as it doesn't make as much power at any rpm as our 268XPS (even though it's rated as high or higher in power output), but cuts equally as well and as fast in smaller logs......Cliff