What do you mean by legal? Are you talking about the size limit? Because the 29" one is certainly within all dimensions with quite a bit of leeway for the wheels... Unless they're more than 7" themselves, which seems excessive to me? Maybe I'm wrong, but I assume most wheels add less than 5"...
I have a lot of traveling planned within the next year, so I need a variety of sizes. At least one big one, for the initial move, a mid-size one for traveling with once I'm over there, and a new carry on wouldn't hurt, though I normally use my backpack. Everything I don't use myself will be going to my niece for when she comes to visit me anyways, so a set seems ideal.