Hey Ketab congratulations on your purchase and welcome to the community. Actually just seconds ago I posted this thread:
http://stackideas.com/forums/what-is-your-favorite-template-framework-and-why which I hope will create a good debate featuring comparisons of the different Joomla Template Frameworks, as I am in a similar boat as you, I feel overwhelmed with options and I am having a hard time choosing my base. You should keep an eye out on the developments.
That said, based on my experience so far, I have subscribed to JoomlArt, RocketTheme and YooTheme, and have worked on sites for clients or friends where they used GavickPro, JSN and JoomlaBamboo.
Here is how I would compare them based on my experience:
JoomlArt: By far one of the most respected, they are continuously trying to implement the latest web technologies. This is great, but it can sometimes result in templates being challenging to configure. If you choose a template with very unique features or animations, make sure you use the Quickstart and read the documentation, as they really have used out of the box thinking to make their themes do wonderful things.
RocketTheme: By far the most feature rich and easy to configure templates, but some would argue that they can be a bit bloated... and I am not a fan of their latest designs.
YooTheme: Arguably some of the nicest designs out there for Joomla and easier to configure than Joomlart, but a bit more complicated than RocketTheme. In many ways one of my favorites.
GavickPro: GavickPro has very well planned out templates and they are also a joy to configure. Better 3rd party component integrations than the rest, they put more effort into it I feel.
JoomlaBamboo: Their templates are stylistically absolutely stunning, but they are less universal than the others.
JSN: I find them inconsistent... that said, they do focus a lot on third party integrations which I really really appreciate... for example they have the only nice looking Ohana supporting template on the market, and they were one of the first to attempt to make a template which revolved around EasySocial. I love their business model however, as they provide limited versions of their templates for free, so you can test before you buy.
If you are feeling adventurous, goto Themeforest.net then go the the Joomla section under CMS. There you will find very unique designs from smaller design firms or even individuals... consider it more of a marketplace than a template provider. There is always amazing stuff there!
Hope this helps.