[Leeroy (Lee)]

Hi folks..
Well i'm in the marklet for a DSLR camera for astro and a little terrestrial.
I've been reading online for the last 3 days trying to decide what to buy but i'm still not sure. I was initialy leaning towards the 550D because of the HD video ability with full manual control over the video settings like shutter speed, ISO etc. I figure this would be great for planetary immaging (in any case a lot better than my neximage), plus an 18mp sensor and liveview etc for long exposure DSO..
That was until i seen a dark frame comparason between the 550D and the 500D, with the 500D having slightly less noise. Not realy surprising since it's only 15.1MP. The 500D still does full HD video but with slightly less FPS (only 20 FPS, 550D is 30 FPS) however there is no manual control over the video settings. All automatic shutter speed ISO etc.
How good are the auto video settings?. Can this be circumvented by using the Liveview to my laptop and adjusting the settings there?

If the 550D has a slightly worse dark frame noise, would it be improved if you selected a lower resolution? Say shoot at 15.1 instead of the full 18Mp?
I've spoken to several camera shop "experts" and none seem to know much about astro stuff..
My budget is limited and i realy can't afford to buy the wrong thing..

Cheers
lee