What is the Summicron's age and condition? Is it a solid mount or a collapsible mount. If it is a good, clean lens I'd buy it and save the $100. The solid mount 50mm f2 Summicrons are a bit better than the collapsible mount versions in my experience (I own 2 of each type).

The 50mm f2 Planar is also a great lens (based on personal experience). It might perform a bit better than older Summicrons in high flare situations. In other picture taking situations, I doubt you could tell the difference.