A generator is only good if you're home for sump pump. I'd first invest in a battery backup sump pump. DH installed ours and total cost was a little under $250. We went with the Basement Watchdog brand. That way we should be covered if not home for not only a power fail but also if our main one was to just wear out over time.
I did a lot of research on generators and we were able to get by with a 3500w unit, we paid under $500. We determined that size as it could power the following at once; refrigerator (or chest freezer but not both at the same time), one TV, Internet/cable modem, uverse TV box, space heater or fan, and charging phones/iPads. Or in the winter it'd power our furnace. Ours doesn't power a lot but for us it works, I wasn't going to invest thousands in a generator which we will hopefully never use. Plus in winter I'd have it run furnace then disconnect furnace and goto TV.