This is with respect to the anna nagar batch... I had attended IMS in 04 and TIME in 05. General comments.. don't know if they still hold..
IMS
+ives
Well structured. They'll give u the entire course schedule & timetable on the joining day itself. Had clear cut separate batches. Doesn't encourage inbetween joining. U'll get to know ur batchmates properly.
-ives
Student quality. The questions provided were either too high level or too basic. Just 8 SIMCATs. Faculty mostly part time fellows, usually iim pass outs, but i wouldn't recommend them to teach!
TIME
+ives
Good quality questions, more than enuf material for preparation. Plenty of Mocks. Good quality faculty. Most of them experienced in their field. Faculty get rotated every month, so u'll have the privilege of being in Raghunanthan's class at least once.
-ives
Not structured at all. Arbit schedules, most probably given to u the week before. Arbitly classes get cancelled, schedules changed, etc. No clear cut batches. U join in at any point of the month (there will always be ads coming out in the paper saying there is a batch starting nxt week!

), u r put in one arbit batch and then it gets coupled with other month batches.. and this process cummulates over the months.. and finally at the end u'll have 2 or 3 full batches. U'll end up meeting a different set of ppl every month. Since faculty gets rotated, there would be some overlaps, some spillover portions not being covered in the next class,etc.
My view as of 2006 Feb
IMS-Stella-good
TIME-TNagar, Adayar-good
Personally, i got to know much more fundae from the TIME faculty than during my stint at IMS. But i liked the way the entire schedule and manner in which IMS conducted the course.