Hello,
None of the college is best and none of the college is worst. Your decision should be based on couple of factors;
1. Program,
2. Campus,
3. Co-op,
4. Placement.
5. Locality (budgeting basis living cost)
All the colleges have their own hold in different areas. Some colleges have good tie-ups with Businesses and fetch good co-op options and some colleges have expertise in certain programs. For example of you talk about corporate communication program in Seneca, it's quite a good program and not all the colleges offer this program. ST. Clair offers a program named International Trade Management which not many colleges does.
If you are financially sound and need not worry about funds while studying, of course you can pick a college even in downtown or even in areas with less percentage of people v.s.v less part time jobs.
Changing college is quite easy process once you land here. You need to obtain acceptance letter from another college (which must be under SPP college list). Scan it send it to your college and they will transfer the funds. Now certainly they will deduct admin fee ranging from 500 to 900 CAD, please check your Offer letter. Once the money is received by your new college ( and you will need to pay any shortage from out of pocket), they will confirm your seat and issue you enrollment letter. That's it. This process takes not more than 2 to 3 weeks. Also do check the withdrawal last date on your offer letter.
Do not disclose this at the airport to the immigration officer as this will impact your entry chances. You can change the college once you have landed here.
Changing college or level of study doesn't require any change in your study permit, hence you needn't inform CIC about this.
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=487&t=15
