I've had a chance to put the GL and Opto Rivers (both 175) through their paces pretty well. They are both nice in their own ways, but they are very different.
The GL is working perfectly for me as my primary fairway driver. It is like a QJLS with a bit more glide, essentially making it more effortless with a bit more range. It transitions very slow and smooth and generally holds it's height well. It holds a turn easily and comes out at the end. On a flat/slight hyzer throw it flips flat with a glidey TB-like fade. I've also been able to throw it dead straight with very little fade on a very controlled flat release. Basically it flies like a dream...It's too bad I didn't get this disc when I first tested the River...
The Opto is a strange animal. It flies VERY similar to a straight-flying domey yellow Striker I had. It flies on the same general lines as the GL, but it's not smooth at all. Instead of easing into it's high speed turn it just seems to kind of pop up all of a sudden. While the fade is kind of late, it is very harsh with the disc getting almost vertical and spiking into the ground. It also seems to like to lift and get air bounces like a lot of really domey discs do. This adds to the feel of the glide, but at no point does the disc ever feel locked onto it's line. This disc also goes really far...Considering the speed at which it needs to be thrown not to fade excessively, this is a disc that needs 360'+ to get it's intended flight path. Out in the field I could crush this disc nose down and hyzer and get some huge hyzer flips...I was putting both well over 400' with a little wind behind me, though in more neutral wind conditions the Opto was going up to 20' farther.
Long story short...The orange GL River is in the bag. This is the disc I wanted the River to be when it came out. In terms of speed, glide, range, HSS, and LSS it is the perfect versatile fairway driver to stand alone in my bag and bridge the gap between mids and my PDs.