This thing kicks the teeth out of the gsf, I can only comment on the 600 tho as never ridden the 1200 so wouldn't know what they are like. But here is how think they compare.
600 Bandit: good bike, goes like stink but all the fun is at the top end and not much at the bottom thus you spend most of the time screaming the tits off it. Great handling and flows into corners brilliantly. Light and nimble which is good for manual handling and they are a nice low commuter.
XJR1200: Fantastic, oodles of low down grunt, long drawn out gears that seam to never end, you can simply chuck it in top gear and cruse down the road on tick over at 60, but then open its throat to go into full hooligan mode. Cornering is a doddle and it sticks to the black top like shit on a blanket. Tad on the heavy side when at stand still or general manual handling tho. Very top heavy with a full tank too.