The A720IS is probably the best bang for the buck in terms of picture quality and out of the box functions... S5is is going to give you better much video and audio.
The A570is should also be looked at--a nice, slightly smaller camera. But it is also the original "WindRiver OS" that was originally supported by CHDK. It supports both auto and manual loading of CHDK (A720is only supports auto-loading). Also, the video compression/quality options are supported in WindRiver (A570is) and not yet supported in A720is (and other "Dry OS" cameras).
There are a few other buried differences between WR and Dry OS cameras (long file name support in WindRiver, 8.3 by Dry OS)--for the most part, it probably does not matter to me (and I purchased the A720is)--but depending on your needs the WR/Dry OS may be of issue for your uses...
? Things are constantly changing--so you may need to do a bit of searching here in the support forum on particular cameras for the exact features you need (like manual focus, or a rotating screen, and--my preference--AA NiMH batteries with option to use AA Lithium for very long life/cold weather use).
In the end, price may be very important too... Would you prefer two cheaper Axxx cameras or one SDxxx--would you worry more about leaving the expensive camera out when doing time-lapse and such.