Once you ride for awhile you'll get a sense for how hard you can lean on the battery depending on the length of your ride.
BTW, spare full sized batteries are really expensive. Shimano is $700, Bosch is $900. I'm sure that Yamaha is similar. The batteries actually have a lot of electronics built in, they aren't like a car battery.
If the regular, high capacity battery gets you "out there", where you run out of juice, I'm not sure what the cheaper, lower capacity battery is gonna do for ya in getting back under battery power.
The trick is to ride the bike and get a feel for real mileage numbers in each Power setting, so there will be no surprises.
As poster Honkinunit said, mainstream batteries are a considerable investment. And there are no mainstream manufacturer's batteries out there that are low in capacity. And because of that, you would be paying near a grand for a 500wh battery in those brands. 475 seems quite low, and I do question the watt hours of that battery and how far it's gonna get you away from homebase. Good luck and keep us posted as to what you do.
Not a bad looking bike. And $475 for an extra 778wh battery seems to be a nice deal. I'd be tempted to go with the extra battery. Impressive Bafang motor specs, too (160nm of power). Definitely looking !forward to reading your ride reports and overall impressions
That bike should ride very decent without power. Another battery might be good if you plan on any long trips where you want to cover a lot of distance in one day or go places where charging points are far apart. $475 is a very good price depending on battery technology.
I ride a lot of areas with loops and shortcuts and not always linear so if my battery dies even one with less capacity could get me back. New to these and just running ideas out here among the experienced eMTB riders.