I have been brewing with 50 litre kegs that were borrowed and now I have my own setup built with American, 15.5 gal (58.6 litre kegs). They both work nearly identically and you can fill two, six gallon carboys with either setup. We do use Beersmith and the software accomodates any values you enter.
There is a little more space to avoid boilover with the larger kegs. My guess is that the empties weigh about 20 pounds. In theory, you could brew an extra gallon or two with the larger ones. Either way, I have to plan my day so that I don't have to lift a full keg at any point if I am working alone; it is too heavy with my bad back. I run about 1/2 of my wort from the mashtun to the boil kettle, lift that up to the burner, run the rest of the wort into another kettle and lift that seperately. Then add that to the boil kettle. Then I can rack or drain into the carboys.
