I would recommend the 6 gallon unit, although either will perform just about the same. With wine, during fermentation you'll see little yeast build-up on the top (krausen for beer brewers) and don't need much headspace. After the wine has fermented, although there should be CO2 inside the empty space at the top of the carboy, having more headspace means there will be more contact area of wine to the gas in the top. In theory, if air is introduced to the carboy at some point (taking off an airlock or bung, stirring to degas, etc.) , there will be more air contacting the wine in the 6.5G carboy than in the 6.
If you plan on long term aging/secondary or tertiary for the wine, it should be done in a vessel with as little headspace as possible. For primary fermentation, it doesn't really matter.
