Hello, welcome!
So you are the smart kid
Looks like as you not only won the voucher but also made a clever decision.
The 650 has not only a bigger screen, it has also SDcard slot to expand the memory (on board memory 2GB) up to 32GB and plays audio. Both reader are good for at least 1,200 book in their internal memory. Mine now have 570 books and still 1GB of free memory.
The small one fits in a shirt pocket, the bigger one not