I have learned SolidWorks all by myself from Video Tutorials and willing to learn another software, I do have VTN tutorials of Inventor and CATIA but can't decide which one i should learn and why?
it depends on your goal. if you are looking to work in the automobile industry then I would recommend you to try CATIA or NX if you have access to anyone of those two.
Also, you should look for people in the industry in your area and asked them which software they are using. then you will get the clear picture.
Inventor is very similar to SW, only has somewhat different command names for the same things as SW has. You can do the same things in Inventor as in SW, so, if you want to learn something different, I think that Catia would provide a wider spectrum of things you can do in the program. The decision has to be yours.
I think that if you focus properly, you can understand 90% of the functionality in the first week. You can download the 30 day trial version for free from Autodesk site. http://usa.autodesk.com/autodesk-inventor/trial/
I had some issues with Inventor 2010 and when I tried to do a comparison test with SW2009 it complained I need a 512MB workstation GPU, at that time, I still had Quadro FX3400 which has 256MB RAM and it was painfully slow on my machine. I gave up because I didn't need to have more than one software package for the work I do and SW is more than enough for me. It cost you nothing to try Inventor and see if you can find it to be to your liking.
I guess CATIA still tops the list of most used software in India ( not based on any stats but by my observations in most job posting requirement ) . And in the automobile sector its almost used in every company here . . .