Comparing the stats of the two cameras as listed on
www.dpreview.com, I can't see much (aside from 2 extra MP on the SX20) that separates these two cameras; same sensor, same zoom range, same ISO range, etc....
Just adding more MP to a camera's CCD doesn't actually make it any better; if anything, it reduces the Image Quality (IQ) by making it a little more grainy at higher ISOs. However, it's usually not that big a deal....
The only thing I can think of is that the SX20, being the latest, is being more aggressively priced to get a better market-share.
I'm fairly sure you'll be happy with either camera, so buy the cheaper one!