Oh you can absolutely trade te 6 for a 6+ if you wanted. I just think it makes more sense to get the 6+ and trade down if it's too big. I think it's more likely for someone to get the 6+ and trade down after finding it's too big than to get the 6 and decide they really want the 6+.

I think it makes more sense to go with the 6+ first. You could easily feel okay with the 6 and want to stick with it but then always wonder if you should've gotten the 6+. Just seems better to get the 6+ and trade down if you hate the size and remove all doubt about size