It's best to try out a thread before investing in a lot of it for a longarm.
I tried several and found I like Glide poly the best in my Voyager (
http://www.bobbincentral.com/glide.html ). Glide is a 40wt polyester thread, does not break or shred on me, is not linty, gives my quilts a little sparkle, and allows the quilt to remain soft even though I have a tendency to quilt closely. I did take a chance on this thread and ordered a lot the first time, since so many quilters said they liked it, and took advantage of the discount they offer for first-time orders. I have not been disappointed. However, I have since seen an occasional post from a longarmer saying their machine does not like Glide.
Before Glide, I tried the poly from Connecting Threads. That is a 70wt thread, however, and did shred and break on me occasionally in the machine. I'm glad I purchased only one spool of it to try out.
King Tut worked well in my machine, but it leaves a *lot* more lint in the bobbin area than Glide, so it is not my favorite.
The former owner of my Voyager said that Aurifil 50wt does not work well in my machine (this is my all-time favorite thread for piecing and quilting on my domestic machine!), but that Aurifil 40wt does work. I haven't tried it, though.
Polyester thread is not necessarily stronger than cotton thread these days. There is really no reason to not use poly.