There is an review of the HD-A20 on C/Net (http://reviews.cnet.com) which mentiones how the two compare. The main difference between the two is that the A20 gives you 1080p output, while the A2 only does 1080i. However, if your set has 1080 native resolution it will de-interlace the 1080i and what you will see is 1080p. Unless it does a horrible job of de-interlacing you *won't* see a difference.
The review also mentions that they were kind of underwhelmed by the A20's 1080p performance, and their "Bottom line" was:
The Toshiba HD-A20 is a perfectly competent HD DVD player, but it's not worth the extra money when compared to the budget Toshiba HD-A2.