The major difference is that the A10 is 1.3 Megapixel, and the A20 is 2.1 Megapixel. I will note other minor differences along the way. There's really only one thing to comment on here, and that's the lack of rechargeable batteries.
Digital cameras drink alkaline batteries like crazy and they end up polluting our landfills.
Do yourself and the environment a favor and pick up a set or two of NiMH rechargeables and a fast charger. Some other camera manufacturers should take a lesson from Canon. They include an 8MB CompactFlash card with the A10, which is only 1.3 Megapixels. I just reviewed a 3.3 Megapixel camera which also included an 8MB card, when it should really have a 32MB card. Neither Olympus or Fuji's similar cameras support either of these options.