Upgrading from EOS1000D

I recently bought myself the Canon 1000D body and several lenses ranging from the nifty fifty to a pretty decent Sigma 17-70.

I have been offered a Canon 550D body and am seriously thinking about pulling the trigger.

What realistic assumptions should I have about the upgrade? Will I see much difference in picture quality? Focusing? Sharpness? The video ability is of course great.

Or should I just skip the 550D and go for the cheaper 1100D?