I doubt it matters.
99% of the time, the people reading your letters will have no clue who the writer was or their credentials. If you didn't specify years of practice, I don't see how they would even know, let alone care. A physical therapist fresh out of school and working is still a licensed physical therapist.
I would think that the level of personal detail of your character and work ethic in academic and clinical settings are what they care about.
I've known people who wrote their own letters and just got their teachers and pts to sign off on them, and they were told they were great at their interviews. I also commonly hear of highly reputable professors and therapists writing really boring and impersonal letters. I found out that one of the major professors in my undergrad program would actually send out the same letter and just change the names. I can only imagine how many people were screwed over by that.
Remember that everyone is going to have a similar lor stating how hard working and dedicated they are. The level of personal detail in the letter is what differentiates it, not necessarily who is writing it.
To directly answer your question, get whichever therapist can give you the most enthusiastic letter with the most personal information about you, regardless of age or years of practice. It helps to have a large amount of observation hours associated with whoever writes your letters just to show the committees that your letters are legitimate.
This is what I assume and what I am basing my strategy for helping the people writing mine to write the best possible letter they can about me.
