25II has more power, dedicated subwoofer out, a few extra features like additional video switching and binding posts for all speakers. Google them and you will find the manuals at H/K support website.
I would prefer the 25II just because it has the sub outs, but I think you have to go by the quality of the used item here. These are older units now, beware the ones that have been abused/overheated.
Both have good amplifier sections and will drive almost any speaker you can throw at them.
I have a 25II I bought new back in the day. The center channel amp died last year, first problem it ever had. I bought a secondhand model for parts from local seller and replaced the amp myself.
Mine still sounds great! We have it driving a set of old Energy E:XL speakers for home theater use. We used to drive a pair of Advent Heritage speakers with it (difficult for some receivers to drive due to low impedance), it never had a problem with those.
Of course, these have no HDMI so you only get pro-logic from them...
Good luck with your purchase.