Walker is obviously a better conservative. As much as I dislike Kasich (relative to Rubio), Kasich would make it more likely for Trump to win in November, than Rubio, who couldn’t even take his home state.
Yes, think of all the Democrat crossover votes that ticket would draw. It would be like having Hillary Clinton in the VP spot minus the Benghazi and e-mail scandals. Of course there is that question about Kasich being Gary Condit's wing-man at sex parties, but who knows. To Democrats that would be an added qualification for the number two spot.