I have the option of loading up either a 36V or a 48V pack in my ebikes as the controller will adapt when the battery is connected. The few times I have tried to cover the same route at the same speed, the bike uses about the same power (watt-hours) . It's not that easy to compare using pedal assist because the bike wants to go faster with higher voltage. So in a sense, your bike seller is correct. Most of his customers will report more range on the BBS01 because they are not going as fast. . However, physics says that if one maintained the same speed, the same energy would be used.
Another consideration is your battery size. A small 36V 10AH pack is 360 watt-hours. A small 48V 10AH pack is 480 watt-hours. Probably not much difference in price or size , but 33% more range. Now if you already have other ebikes, and if it is possible to share batteries, then pick a common voltage.
I would say that the BBS02, ridden slow, will make you happier in the long run. I have one, and ride it pretty slow.It only runs on 48V tough. I did my voltage comparisons on a hub motor.,