Hi alexial, I think it's great you have a professor to write you a letter. I would use just one of your research assistant managers if you can. It would be best to have a variety. I think that it sometimes seems that you've lost contact with past supervisors, but they remember you and would be happy to write you a letter if you asked. I thought for 2 of my letters that was the case but really I just emailed them saying "hey remember me?" and requesting the letter and they were happy to do it. I also had one person not respond to me at all, but that's why you ask a few different people! If it comes down to it and no previous work or field placement supervisors will write you a letter, then better to have both managers from your research assistant lab than one. I suggest you do everything you can to get variety especially considering the competitive nature of U of T.