I'm coming from a similar situation. I bought a ninja 250 last summer to learn on and started looking around for something with just a bit more power thinking I'd get it in the spring. I was looking at the Buell XB9S, BMW F650CS and the Monster 620/800.
I lucked into a lighjtly used '02 Monster 620 Dark last weekend (i think it was luck..
My impressions coming from the ninja 250:
Handling. Like the ninja, the monster is light and handles great - I actually feel more a bit comfortable with the handling of the monster.
Power. The monster has considerably more power than the ninja
Visibility. I like the mirrors on the monster better than the ninja - because of the seating position they are closer to your eyes and shake-free. (note tho that while I'm happy so far with the monster mirrors, most here are less pleased with them and replacing them seems to be a very common mod)
Looks. This bike looks sooo good. People notice it. They like it.
Insurance. It dropped. Not much, but it dropped. Amazing what having less plastic will do to rates, even with the cc increase.
Clutch pull/placement. In my case, the short legs that limited my choices for stepping up are accompanied by small hands. On the ninja, I could adjust the slack on the clutch cable to move the friction zone closer to the handlebars. The monster has a hydraulic clutch and no means of adjusting it. So there will be adjustable clutch and front brake levers in my future.. Oh, the friction zone is very small and first gear is kinda tall compared to the ninja, so stop and go traffic is a bit more work. Of course, that's motivation to find ~other~, less-travelled routes [somehow, with a 6.6 mile commute, I managed to put 400 miles on my bike in its 1st week..]
Gas mileage. I got about 62mpg with the ninja. Now I'm getting 42mpg [some of that ~may~ be the enjoyment of the increased power - heh].
No centerstand. Remember how easy it was to clean your chain on the ninja? Remember how easy it was to inspect your rear tire?
The sidestand. This thing sucks. I hold my breath every time I deploy it (less convenient than on the ninja) and then - as I start turning blue - I turn the handlebars to the left and sloooowly let the bike down onto the sidestand, marvelling that, despite everything my senses scream at me, the bike somehow doesn't just fall onto its side :-[. This sidestand makes me nervous - especially given the lack of a centerstand.
The seating position is a little more aggressive than on the ninja. The pegs are back a little and you're leaned onto the handlebars. My initial riding was with straight arms and I've had to train myself to relax and have a slight bend to my arms. I'm getting uncomfortable after about 45 minutes in the saddle. As I get used to the bike I hope that'll improve.
I hope this helps.
And while I couldn't keep my ninja and have the monster, if you can swing it, I also recommend holding onto the ninja as a 2nd bike if you can. But, if I had to choose only one bike, it'd definitely be the monster