I own a 7910 and I'd recommend you get the 6100, I put a 20" on my 7910 and it didn't even flinch to being buried in balsam poplar, it might've dropped 100-200 rpm when I put it in 18" Elm. I have a 28" on it now and that's what it's for 28-32" in 25"+ wood.
True , my PS-7900 just scoffs at the 20" bar she wears - but she is huge fun and the sound is amazing (with a small muffler mod).
Dogging her down cutting fresh beech tree trunks up to 20" in diameter my PS-7900 held no less then 10,5k in the cut , pure awesomeness.
NOTE: The PS-7900 is the predecessor to the PS-7910 without a CAT muffler and a coil limited to a slightly higher rpm!
Tested out a 6100 and really liked it, but have been wanting to step up a class or two. This will be strictly a firewood saw and wear a 18 or 20 inch bar most of the time. May have the need for a 24-28 inch bar but pretty rare. This being said is it worth the extra $ for a 79cc firewood saw?
I don't have a PS-6100 , but her specs and reputation indicate she'd be a good pick for You considering Your needs.
For the fun factor I'd keep a eye out for a used but not abused PS-7900/7901/7910 , that is how I got mine!