If you are a tool fanatic like me, then you'll need every tool imaginably to do the job right. When the job comes up, that is. And when that job is protecting the world from danger, you'll need a Sonic Screwdriver. These things can open doors, make wiring work, short circuit wiring -- well mainly do absolutely nothing. But it will look cool while emitting a light and making a high-pitched annoying sound while doing it.
The 10th Doctor's sonic screwdriver extends also, but more importantly doubles as a pen. It comes with regular ink and UV ink, for writing secret prescriptions. As wielded by David Tennant in the series.
I honestly have a hard time choosing between them. I guess it boils down to which version of the Doctor you like better. Or who you think is the sharpest dresser. Or if you like green or blue better. Or you could just get both and not have to decide.
I swear I saw other versions, but I can't find them anymore. Probably only on eBay now.