It ALL depends on mindset.
If your mindset is that you will enjoy yourself regardless of weather, play regardless of precipitation or cold, and consider it worthwhile regardless of what you shoot, I say definitely go with winter
If you are choosing winter and pinning the success of your trip on the hopes of catching one of those good weeks, and if you miss and it pours, freezes, or snows, you will request rain checks, get drunk, and curse your bad luck, then do not mess with Bandon in winter. (Really, do not mess with Bandon period, but if you do certainly maximize your odds of good weather).
On my two trips to Bandon, in both cases I have been one of the more gung-ho guys out there, tossing on the rain gloves when needed, braving the snow and hail, etc - and that's fine. Sure Id rather have had bluebird skies, but would I have spent 1000-2000 more for those? Probably not.