I own the T&T 1510-3 and it is my favorite 15 ft rod. It is just a tad (meaning ever so slightly faster) quicker than the 1510-5; but the difference is not worth talking about it is so slight. The 15 ft GLX is a noticeable faster and quicker rod than the T&T 1510 and is a little lighter.
The other difference is that the 15 ft GLX uses one line size heavier line than the T&T 1510. The GLX is a 10/11 rod and the T&T 1510 is a 9/10 rod. I use my T&T 1510-3 with an 8/9 GrandSpey 85% of the time and it is a sweet heart with this line. An XLT 9/10 is also a nice combo on the T&T 1510. If I had the GLX, I would use a 9/10 GrandSpey, or 10/11 XLT.
If you want a 15 ft T&T 10/11 you need to look at the T&T 1511-3. The 1511 is a much more powerful rod than the 15 ft GLX; however, it is slightly slower than the GLX. I opted for the T&T 1611 instead of the 1511 because the longer length provides much more line control and is better at covering large rivers than the 15 ft rod.
I really like the 15 ft 10/11 GLX and came close to buying one until I cast the T&T 1611. If you are going to cast short-belly (Windcutter, Deltas) or spey shooting heads on the rod, the 15 ft GLX is a little better at doing so.