I haven't personally seen a big accuracy difference in the Gen 4 full size guns over Gen 3, but some others have. Could be just a sampling difference. In the baby Glocks, of course, the double spring has been there right along through the generations.
What I'm finding with the G26 is more versatility at this time in my life. I shoot matches when I can, and am intrigued with the idea of a very small carry gun that can hold its own in a match environment. Light recoil and low price of 9mm make it particularly attractive for that. I've shot the G26 in lieu of the bigger Glocks at a couple of IDPA matches and a couple of GSSF matches, and been pleasantly surprised at how well it performs. However, I don't see myself putting my G27s or my G33 up for sale any time soon, either.