I don't think there's a preference aside from ECE majors (since they need certain programming classes—however, they may already have reservations for those).
Transferring into CS is relatively easy if you do well in the intro classes:
- 15-112 (unless you have AP CS credits)
- 15-122 (unless you took AP CS AB before it got discontinued and can get credit)
- Concepts of Mathematics (21-127), or the new CS-major replacement which is, I believe, 15-151?
- the infamous 15-251
A friend of mine did quite well in the above and successfully transferred. If you look on the department website they explain what courses you need to enroll in and complete to be considered as a transfer/double major (I'm pretty sure it's close to the above listed).