Which version of Shovel Knight should I get? I own all the consoles and built a high end pc (prefer consoles though).
The consoles & PC will be getting a local versus multiplayer mode in the future, the handhelds won't.
The Sony versions are cross-buy and cross-play across all three platforms (PS4, Vita, PS3) and feature an exclusive God of War-themed boss fight & item.
The XB1 version features an exclusive Battletoads-themed boss fight & item that's much more elaborate than the GOW content.
If you buy the SK Amiibo you can access "Custom Knight' (lots of exclusive items and customisation options) on Wii U & 3DS, as well as a local co-op mode that's exclusive to Wii U.
The 3DS has an exclusive "Streetpass Arena" side-game; it also has stereoscopic 3D (duh!) and if you're someone who's bothered about pixel filters and sub-pixel movement then the 3DS version is optimal as it has none of those issues that are present in the HD versions.
The retail versions don't have any additional content over the digital versions; you do get a digital code for the soundtrack, but that's already pay-what-you-want so you can go grab it for free right now if you really want to. (I don't know if the cross-buy thing applies to the retail PS4 version.)