Easy answer do you want a lens for it's signature look that doesn't suit all subjects (sonnar) or do you want a more neutral lens that can be used for most subjects (planar). I would personally go for the Sonnar I like it's look, the planar produces sharp images that lack something imo. According to Roger Hicks and many others the newer Sonnars are adjusted to 1.5. from 2.8 on the dof makes up for the focus shift.

Good luck