There are two, almost identical, suits in this category:
The Speedo Solid Lycra Super Proback (lycra fabric) and THIS ONE, the Speedo Solid Endurance Super Proback (polyester fabric). The lycra fabric has a smooth feel and shiny look to it; the polyester fabric is more matte, with a coarser feel.
Both the suit description and the comments say this polyester "endurance" fabric lasts a long time - perhaps longer than the lycra. I found this suit is cut bigger, with a looser fit, but the butt is NOT LINED in this polyester suit.
The lycra suit has a tighter fit and is LINED in the back - Fantastic! Hurray!! - but is very close cut in the chest area (there needs to be more fabric under the arms, especially for the larger sized suits). The polyester suit seems to be cut bigger, has more room in the chest area, and actually fit more comfortably out of the box.
In the end, I went with the lycra. I love the smooth, shiny feel, and although the "endurance" polyester is supposed to last longer, it was a bit too roomy. I had a feeling it would stretch out too soon after wear in the water.
Having just tried it on, I LOVE my lycra Speedo. The suit holds me in and fits well. Though I imagine this polyester suit would have done just as well (especially reading other comments) I look forward to many months - perhaps a year? - of swimming in mine twice a week. And because the behind in the lycra is - once again - LINED, it won't fray and sheer as quickly as my older one. Why doesn't Speedo line the back of THIS polyester "endurance" suit?
Happy Swimming, everyone!