Well, bearing in mind this is a try-out-at-home-as-loud-as-I-dare try out, not against a drummer. But I did bloody crank it..
I'd say the 500 is louder - by much ? I played it though an SWR 2x12 and an Ampeg SVT210AV and it certainly made everything rattle more in my boxy studio, so by that yardarm quite a bit ! I think the 500 sounded better at that volume as well- more controlled. Also, I didn't have to turn the gain up as much with the 500, which gave the compressor a bit more control and over all less hiss.
If you can stretch to the 500 do. The 250 is going back, even though it kindof takes it out of the price bracket I'd set myself.
They're nice-sounding amp's too, overall I'm impressed.
Oh, and the fan isn't loud, which was a worry. It's audible, but it's like a gas fire, not a whine. Quite relaxing.