Obviously 0.5.1.
I think the main reason why people vote for 0.1.3 is because of different gameplay with side-scrolling-only levels.
But then I don't see the point of nostalgia, because ST 0.5.1 is still able to support the classic side-scrolling-only levels anyway, but is also is capable of supporting complex levels.
The big question is basically just if someone is going to make more of classic-style levels.