I have fished both rods for two years while guiding in Alaska with the 8 weight, 9 foot models. For me I think that the GLX is about ten times better than the GL3. If you are really serious about fly fishing and want to have a good rod that will last you a long time. I recommend the GLX. I have fished with G Loomis Rep. for the last two years and he claims that the GLX is worth it when compared to the GL3. The GLX tends to be stightly faster than the GL3 and way smoother. Good Luck!!
I have a 9'9" GL3 in a 6wt. It has a lot of backbone but is not very fast. The result is that I throw big loops pretty far. Its a good rod and I am happy with it though I'm not sure if I would recommend it. I have also cast the same rod in a 5wt and did not like it, way to slow, like a buggy whip. You also have to consider that both those rods are 9'9" and may be alittle slower than the nine footers. If you are looking for a quality rod that is inexpensive I suggets the St. Croix Legend Ultra or Sage VPS. I have a feeling that the VPS is a very under rated rod.
I haven't fished loomis rods, but I wanted to add to the quick mention of St. Croix. I have a UFT996, Legent Ultra 9'9" 6wt. It has been a spectacular purchase, and will always be my backup trout rod. Although I am currently considering a higher end rod, I have no interest in giving up my Ultra. Fast, Crisp action, great guarantee, and great for dredging in deep water. I actually fished silvers this last summer, when the water cooled off and I felt as though I could really lay it to them. Truthfully, I've considered stringing it with seven weight lines. I also have the UFT 998(8wt) that is strung with 9wt lines. I bought the 8 because I was so happy with the 6. I think they are a tremendous value. They are ugly, ugly, ugly, but it isn't a fashion show. I have broken the 8 three times on chums(don't laugh, I now use my 9wt) and they replaced it three times. My only compliant with St. Croix is that I broke their top of the line rod three times. I think they replace broken rods with all of their flawed rods in order to keep cost down. Go cast one, I think you'll consider it as an option. You don't get that guarantee with many rods in the $200-$325 range. They say you'll still have money to buy beer.