I have given VITEEE last year and my friend whose rank was nearly 7k got CS there. One of my friends got Civil, his rank was 13k. Don't decide now as counselling dates I think will be in early or mid July. Let the JEE and AIEEE results come out, then decide.
As with branch, it's not that mechanical means mechanics, it contains Engineering Drawing, Industrial subjects and many other things. Also, Electronics is said to be most difficult branch (I too have that
). Its not that if you know soldering, making circuits or if you are good in semiconductor which you studied in Class XII, then you are good at electronics. Electronics contains many different subjects, which do not have relation with subjects which we studied in past. In Computers also it doesn't means that having good knowledge of programming languages like C, C++, Python, Java etc. that you are good at computers, it includes many other different subjects like Network, Database, System Development Life Cycle, Algorithms etc. which also don't have any connection with those languages ( I didn't mean that Database etc. are not designed/created in those languages but I mean that they have completely different logic which you have earlier studied in those languages). So, you should take that branch in which your area of interest is more.
Good Luck!! Also don't get tensed!!