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There is nothing wrong with 6 volt. Starting, lights etc. If its slow cranking and dim lights, there is a reason...fix it. All my six volt cars will crank for days and the lights are plenty bright enough, especially 6V sealed beams.

Many folks change because they simply don't know how to chase the problem. Correct cables, correct grounds, properly rebuilt starters and generators, you'll be fine.

Thats my rant on this.

I approach this from a slightly different perspective.
If the car is original and parts are available, I'd leave it original. As said if the system is good, 6V works just fine.
If it's in need of parts or major electrical repairs, I'd make the switch. I tend to think long term.
There's alot of mystique around why industry changed, but if you look at battery technology, by nature, a lead sulfuric cell makes a little over 2 VDC. So it's just a matter of how many cells you series to get multiples of 2+ volts. As cell design improved, they could get more cells in the same space. The original 6V's were long and took up much space and weight. Compare with the optima of today.
The other side of this is current density. How much amperage you can get per cell. As I've said before at 6V's you need twice as much current to do equivalent work at twice the voltage. (Power(watts) = volts times amps) Why you need those bigger diameter cables.
Then there's positive versus negative ground and the impacts. The negative side is the cathode and electrons travel to the cathode. So the "body" is supplying electrons to make the conductor path in negative ground (12V) and the anode or plus side is supplying in the case of positive ground (6V) so there's slightly more demand and chance to draw down the battery with positive ground. But a good disconnect on the battery if you don't drive it alot is an adequate solution.
Generator(6V) vs alternator(12V) is another aspect but I'll leave that one alone for this discussion

Thanks Fish'in Jim - that's a very helpful,informative explanation. I had especially wondered about the positive ground. An alternator, rebuilt with regulator (6,12,or 8v) in the generator case can be done. Jay Leno had it done on some of his cars with excellent results by Don Allen (Gener-Nator). It's a bit pricey $900 though, so that's why I'm trying to decide on what approach to take with the car. With 12v, I would always have bulbs and other accessories, but as you say, 6v has many positives.