What is the difference between the 51mm...
What is the difference between the 51mm and 43mm stem, other than length?
The 43 is threaded, were the 51 has a smooth valve stem.
Really does come down to length.
Its just the length of the valve. I always go longer, you never no when your going to need the longer valve and it never hurts to have extra valve even if you have short rims.
There maybe that day your riding and your bud gets a flat and does not have a tube and you need that longer valve.