Hmm, for me it's mostly a preference thing? I think he's stronger building ad and on-hit, namely because of how his q, e, and R work, but I definately know people who can run him ap midlane. Ap has slightly more utility + burst, AD/AS has more dps.

Can you elaborate? I much prefer magical footwear to the other options, because of the sheer amount of utility it provides. It's very consistent and just all around a solid option. For domination runes, Twitch is so item dependent that he consistently gets less out of ultimate hunter than just taking the extra gold. Both footwear and bounty hunter are good because they let him essentially cheat gold, helping him hit those lategame powerspikes sooner. They're not as a flashy as other options, but I find they're very consistent, and help Twitch accomplish what he wants to do in a game.