How big is your microwave? Be aware that the wattage of a microwave oven is the output wattage, and the power it requires is a good bit higher, and often the power factor isn't that great either. A 1200 watt generator isn't going to power a 900 watt microwave effectively...except perhaps if it's an inverter microwave, which is generally better behaved on lower power supplies and which will actually lower its power consumption on lower cooking power levels (rather than merely cycling on and off as other microwaves do).
I would guess that the bigger generator would be more useful to you. At these prices, I assume these are both open frame contractor style generators, so both will be quite loud and bothersome; you probably won't care to sleep with either one running unless your camping at someplace like a racetrack where there's already lots of noise. You won't make many friends running them in campgrounds, either, but if you're dry camping out away from other people that's not a big concern.
