I went with the mark I, as much as I would have liked the mark II it was just too expensive and will probably remain that way for a long long time.
I was initially worried about the lack of IS as well compared to the 17-55 but if your shutter speeds are getting slow enough to introduce blur then you can just up the ISO on the 5DII because it handles much better at higher ISOs and the noise cleans up nicely. I regularly use ISO 1600 without a problem!
Sunday and Monday are the best nights weather permitting when the Perseid meteor shower is at it's peak with a predicted 100 meteors an hour.
It really depends on the effect that you are after. If you want nice blurry star trails with some streaks of meteors then I highly recommend this excellent guide written by Matt as a place to start.
If you want each star to be visible then the generally accepted formula is 600/focal length on a full frame or 300/focal length on a crop like the 600D but I have yet to put it into practice!
Thanks Rico. The article is really useful. I am hoping to get out on the Moors somewhere to have a go and see how they turn out on Monday night.
