My Farmall H is converted to 12 volts. If you do it right, 12 volts is a great conversion. I see a lot of positives to it. The reason I say "do it right" is because many times I've seen it and it's a complete cobble job, wire splices everywhere and sometimes even holes in the hood to mount an alternator because it doesn't match how the generator bolted up. This is how the alternator was mounted in the same place as the generator would've been.
I really like what they did. The brackets are a little odd but as you can see they work really well and hold the alternator there without the need for a single hole in the hood. The only thing it interferes with at all is the one spark plug wire and that's not really a big deal. As for the wiring itself, the messiest thing I can find (which you can see in the image) is the ballast resistor zip tied to the coil. I can easily remedy this by replacing the coil with an internally resisted one.
Obviously when doing the conversion, you'll need a 12 volt coil. Probably a 12 volt starter, though the starter on my H looks like factory. Not sure that it is though, as I'm pretty sure 6 volt starters won't work very well with 12 volts. Of course you'll need to make sure to change the wiring to fit the alternator you're using.
As for pros and cons, there's a lot of pros in my opinion. 12 volts allows for much stronger engine starting as the starter will be more powerful. It should also start a lot easier in the cold, my H has been good even when starting in freezing temperatures. Lastly, as long as its wired correctly, the alternator will do a much nicer job charging the battery than the generator would, since the alternator will still charge even when idling. I have a functioning ammeter on my H and the alternator charges the battery very well.
The only big con I can see with 12 volt conversion is the simple fact that it's taking an all-original antique tractor and making it not so original, which is something I really don't care about but a lot of other people might. It may also take a while to sort out any wiring issues you find while working on it, especially since the machine is still original 71 years after it was built. I suppose finding the best parts to complete the conversion might be a con (especially if you need to locate a 12 volt starter that fits the machine).