If you are just bashing around I would run the #7.

A #6 is a hotter plug and unless you are sure your engine is shimmed properly you may cause damage with a #6... unlikely but possible.

With a properly shimmed engine a #6 would be fine @ 50F. I believe it's recommended to run a #6 in temps less than 60F although I've never had issues with a #7 running in temps as low as 50F on cold mornings at the races.

The #7 is a safe bet