The main problem with a wire floor is that it makes it suddenly real difficult to move the tractor. Normally, in a floor-less tractor, they just walk along inside as you move it; but they can't do that with wire, and indeed risk getting squooshed feet when you put it down. So you have to either lock them in the floored part for the move, which makes a rather bumpy and insecure and stressful move for them, or you need to remove them from the tractor completely. Also a wire floor prevents scratching (which is a really important part of chickenhood)
In my opinion, the best solution to keep predators from digging in is a) shut the chickens into a floored house portion of the tractor at night; and have an apron of wire mesh (preferably like 12" or more wide, but, anything is better than nothing) all around the bottom edges of the tractor to discourage digging.
(e.t.a. - I'd be real leery of just parking the tractor on a piece of mesh. It still prevents scratching which is kind of tough on the chickens, but as many predators are quite a lot stronger than you might imagine, it may well NOT be preventing a predator from getting in - all they have to do is tip the tractor up enough to get under the frame, or push it enough that it slides partly off the mesh. Everybody underestimates the strength of dogs and mature raccoons. Don't. Especially when they have a chicken dinner in sight.
Good luck, have fun,
Pat
