Xv01 I think is a bit trickier to build mainly because it uses machine screws rather than self tappers. You can get away with just ramming them in but it’s nicer and stronger if you tap the holes first. Make sure you get a roll tap as a cutting one will end up taking too much material off. Also need to take care setting the mesh and the belt tension, but nothing else is difficult.
Not had a tt02 before but the XV01 feels more like a real car as the motor is at the front. It’s also a lot more complex with loads you can adjust like oil weight in diff and pretty much fully adjustable suspension geometry.
In terms of the body, it is easy to spray as it is single colour but the decals are tricky as there are just so many of them and the large ones go over quite a few curves and require stretching.
If you are not happy with that, you can get a killer body pre-painted one which I heard good things about. I am sure you can get most of the cost back by selling the original shell on.