Hi ,
Right now you concentrate on preparation, regarding choice of branch,  the best branch would be dat in which u have interest. Yes , your preparation for jee suffice for BITSAT. You can give one year to preparation for IIT and with hard work and guidance you can easily make it to the IIT's, You can join online coaching as it is flexible n you need not to travel anywhere n you can utilise the saved time in studies.  For this you only need Internet connection.
You can join  www.Askiitians.com  as askiitians is India’s first educational portal offering completely online-interactive-realtime 
resources and video solutions for preparation of IIT-JEE and other engineering entrance exams.



Here you will get online-interactive-realtime learning sessions, problem solving sessions, strategy sessions, counseling sessions from IITians.
There are many packages available in askiitians.com but i wud advise you to join course for one year .i.e. RIP1
And for more information regarding packages, click here :www.askiitians.com/packages.aspx
Thanks and All the Best for your future !!!!