This subject has been raised many, many times before. . . .
I'm a big fan of the Pratt books, but it really boils down to your preferred reading and learning style. If you're more of a 'visual' person (ie you prefer pictures, diagrams etc rather than text) go for Pratt; if you prefer all the words a few diagrams thrown in, then go for Thom.
If you do go for Pratt, BUY THEM ON AMAZON!!! They're much cheaper than elsewhere, High St or internet. . . .see attached link (may need to copy and paste this into your browser).
http://www.amazon.co.uk/exec/obidos/...333645-4284654