No doubt... But whichever way you go, both projects will prove quite interesting and unique, I'm sure!
Any one of those projects would be interesting.
The gokart would be sort of like Simon Thomas's(I like the looks of Simon's cart) cart in that it would be almost square at about 4' but with full suspension.
The minibike would just be a bit on the wicked side with that 2-stroke and it may get built anyway for the OMB build off.
The tractor is my pick and would most likely cost the most but I really think it would be cool sitting there on 6 18x9.50-8 or maybe even 21-11.00-8 lug tires.
The 6 wheeler would be a fun all year round toy.