Why do you switch from SVN to Git?
GIT is really much better, in fact after using it everyday at work I can hardly work with SVN anymore. It would be really long to tell you why it's better, I guess you can find such comparisons with Google; but if you don't I'll try to elaborate a little bit
The most important thing is that it's a decentralized system, which means that there's no server, only clones. So people (especially me) can work locally without pushing anything to the main clone, or can fork and work on their own clone and then submit a "pull request" (which is a request to merge their clone to the main one), etc. Branching and tagging is also much more flexible and lightweight.
I'm not too familiar with Git, but it seemed to me rather complicated compared to SVN (which I consider a very nice version control system)
In fact GIT is a superset of SVN (you can even use GIT on top of a SVN repository directly). So at least you can do exactly the same as you did with SVN, and if you want more you can learn a little bit about the extra commands and do more powerful stuff.
On Windows, TortoiseSVN is also a great tool, is there something similar for Git?
Of course, every version control system has its Tortoise app
Here it's TortoiseGIT, and it behaves exactly the same as TortoiseSVN -- maybe too much, sometimes it's confusing because it keeps SVN concepts unchanged but they don't match GIT anymore.
Isn't SVN also used more widely? I personally don't know a lot of people using Git.
In fact I think that GIT, and decentralized systems in general, are getting more used and will eventually replace SVN one day. Because decentralized systems are really more powerful.
Have a look at github and gitorious, you'll see plenty of well known projects using GIT.