I own a 128GB iPad Air with cellular on Verizon. I see no reason to upgrade this upcoming update, nor do I ever see a reason to upgrade from one generation of just about any Apple product to the next (unless we're talking about the iPod touch, which doesn't get updates often enough).
I will, however, be upgrading (replacing) my first generation iPad mini this coming update, as the A5 processor sucked with iOS 7 and sucks that much more with iOS 8. Plus, despite it being a two year age difference, the CPU will be three generations newer (assuming it gets the same A8 found in the iPhones) and that is quite a speed bump.