Not going to make a difference.
Someone here already measured the charging as maxing out at 3 watts.
Meaning even a basic 5W brick can provide more than the watch will draw.
Whether you use a 5W or 12W USBA source, or buy the USBC charger and use an 18W source, you likely won't see a charge speed difference.
Remember - we're talking about a 1 watt hour battery in the watch.
EDIT - you might choose to buy a USBC version for convenience if you're charging from your macbook most often, or if you have standardized on USBC for your travel kit, but there's not going to be any other benefit.