You're rather comparing apples and oranges when you compare the Sony Reader with a Tablet PC

. If you want to read
fiction - just normal books, from cover to cover - and have a device that's instantly "on" and you only have to recharge once a month or so, the Reader is unbeatable.
If you want a full-blown computer
or if reading A4/Letter PDFs is important to you, a Tablet PC would be a much more sensible choice.
Better still, get both

.