To me this is the classic Bill Clinton question "it all depends on what..." value is.First of all value could mean show car value. If your car is a high grade show car with everything exactly as it was in '55 but with better than new, stock colored, paint and upholstry in accordance with the data plate then it probably would sell to a real collector for more with 6V than 12V This mostly applys to 55 T Birds because there isn't quite the show circuit or market for '55 Fords.
But if your idea of value is being able to drive the car when you want and get home again, and if it has virtually anything that deviates from stock in any visable way, then the value of the car is likely to be increased by you driving it more. Your enjoyment will certainly be improved by driving it more and that also encreased exposure to the public will therefore increase its sale value as well.
As mentioned above you can always convert it to 12V and keep the 6V stuff for future sale. But I would doubt that you would ever go back.