The buttons and joystick are a definite improvement. Performance-wise, the ipad air 2 will be hugely better!
I have a friend who converted an Ion iCade to a desktop cab, mounting a replacement ipad screen in there, and running it with a raspberry pi 3. It works well, but he removed all the original joystick and buttons (too clicky) and replaced with arcade joystick and buttons more comparable with the Picade ones.
So I’d say yes the controls in the Picade are noticeably better than the ION icade ones, the picade cabinet is also a bit lighter, and a lot easier to modify (swapping out buttons/joystick, speakers, lights etc). Also the Picade has perspex over the screen and marquee which make it look more like a proper arcade machine.
The ION iCade is easier to add vinyl side-art to though - there’s sellers on Ebay that will sell exact-fit artwork. On the Picade, you are quite limited in what art you can add, because of all the screw heads which aren’t flush with the side of the cabinet.
The other nice thing about the ION iCade is the ability to stick a bluetooth woofer behind you, and it works fine. I’ve tried to do something similar using the picade running retropie and never got bluetooth speakers to work (it detects them, but nothing outputs sound to them).
So I’d say if you are unhappy with the quality of the controls on the ION iCade, the Picade is better, otherwise, perhaps see if you can find someone local who will let you try it before deciding whether it’s worth shelling out £180.
If you like tinkering about with your cab and changing things, then the Picade is a no-brainer, it’s really excellent for modding/tinkering with!