I have yet to try the Tretorn - though I have read many good things about them.
Though it is important to note, Tretorn does make a few different types (pressureless and regular):
The Micro X are the most popular and are an entirely different construction when compared to other pressureless balls.
I personally have not used them, though I plan to when my current batch of Penns die - I bought two buckets of the Penn pressureless from Amazon.
Ive read the Penn will last longer when compared to the Tretorn - though the Micro X at least, are a lot more arm friendly and feel like a regular ball (when hitting).
Ive been using my Penn pressureless for about a year in my ball machine and they are still going strong.