I had the Hexanon concurrently with the latest Summicron, and found the Hexanon a tad less good in the corners wide open, and a bit less good in big enlargements (1620) when it came to very fine detail. The Hexanon also showed slightly less contrast, a good thing if you're shooting chromes. The Summicron is absolutely top-notch, but if you're printing smaller than 1417 I'd say either should be fine. Note the Hexanon takes bigger filters (40.5 correct?) if that figures into your decision.
I never noticed much difference in flare, even going toward the sun, only using those little pull out hoods, but I don't shoot that direction a lot.
In the end I sold the Hexanon, tried the Planar, and ended up with a second Summicron.