Personally, I'm madly in love with the 50/1.5 Sonnar, but its big strengths are in intangibles like "character", bokeh, color rendition, and so on, rather than incredible sharpness. I've never had a Summicron, but based on reputation, testing, and the results of others, it seems to be clearly the sharper lens of the two. Nevertheless I can't imagine that any lens, whatever magic it might have of its own, would *replace* the Sonnar for me.
The only Planar I've shot is in medium format, which I believe is the same optical formula as the 35mm versions. Sharp enough to cut yourself on, to be sure, but it's hard to compare lenses of different formats in apples-to-apples terms (apart from quantitative bench tests).
In any case, I think it's safe to say that "Sonnar or Summicron?" and "Sonnar or Planar?" are both good problems to have!