And I'm sure that the shop sells it, too, don't they?
My suggestion is to use what you like. If your machine doesn't like it, it will tell you.
I have a Bernina QE 440, and I've always used the Gutterman 100% cotton (50/3 weight) for piecing. I've used rayon thread for topstitching, and I discovered the Prescencia 100% cotton (60/2) threads (which I love and which is replacing the Gutterman!). All of them work just fine on my Bernina. I know one 440 owner who won't use anything but the Mettler threads and another who loves YLI.
It's my understanding that most Aurifil threads are a 50/2 weight, which is a little light for piecing. I like a 50/3 or a 60/3 for piecing and for quilting, which is why I use what I do.
For more information about the differences in threads, Harriet Hargraves' "Heirloom Machine Quilting, 4th Edition" has a lot of good information.