Check hubs, spokes, and rims. If all are the same, I'd go for the $280 version. I suppose if you pay an additional $60 (for the pair?) for tensioning, that brings you up to $340, and you still save $60.

Are the "machine built" wheels truly machine built? Or are they just machine assisted?

Anyway, if they are using name-brand spokes (DT, Sapim, etc), I'd go with the cheaper ones.

If they use generic spokes, skip it.