I have a Mosaic that I have been comparing to the Pronto. I have taken them both around the office and let people play with them both. Everybody likes the Pronto more. Even those who have never seen either one before. Here is a list of reasons why the Pronto is better.
1. Pronto is smaller
2. Pronto Looks better (my opinion)
3. Much harder to activate the Mosaic touch screen buttons. Plus, you can't "punch" at the buttons with your finger to activate them.
4. The hard buttons on the side of the Mosaic are not as easy to press as the Pronto's.
4. The Mosaic's backlight isn't as nice.
5. The Mosaic's display and the graphics aren't as nice.
6. The code library is nice but Pronto will have that soon.

At the same time I think the Mosaic will do just fine because it is cheaper and has a faster and easier setup. Like most things, people who learn the Mosaic first will like it better. But for those who like to tinker with gadgets, the Pronto is great fun.