Hi, I thought I would chime in because I just got into a NP program and spent a lot of time getting as many quality letters as I could.
I think that a good letter is as important as *who* writes it, if not more so. If you know the TA on a personal level, but the prof is willing to sign, that sounds like a great deal. Good letter, good authority.
By your first post, it sounded as though the prof was just reminding you that an "A" is not the only important detail in a letter. ... In fact, these letters don't usually mention a grade, but what sort of student (ie motivated, inquisitive, etc) he/she is. I didn't see anything negative in his letter, it just sounded like he is the straight forward type.
I think you should give the TA as much info about you as possible so he has a lot to work with when writing the letter. Maybe even offer to take him out for coffee, or to sit down and write it together so you can make his job a little easier. The more info you supply him the better--resume, list of acheivements, list of personal qualities, academic & nursing goals, etc--that way the TA will have more to talk about and he will also see how important this is to you and probably spend more time on it. ... The more time/info in that letter, the more weight it will hold up to the admin commitee as it will appear that he knows you well.
I would also ask to see the letter before he hands it over to the prof. A lot of people do this and I think it would be a wise idea if you have any doubt he may not write an excellent letter.
You are right--the prof is not going to write anything crabby or neg, people that agree to write letters do not do that & know how imp they are, he is probaly just going to sign it and drop it in the mail honestly. He probably just offered to add something to be polite but really doesn't feel like doing any of the work.
I say, go for it, sounds like a great deal!
Best of luck!