There is a bit of a difference between the questions you are asking. On one hand you ask who has more kilts, then on the other more kilt wearers. Two somewhat different questions that might yield different results as one kilt wearer does not equal 1 kilt (Hamish is the perfect example). As has been noted here before Scots tend to have fewer kilts, usually in only one or maybe two tartans, if they even own one at all. A lot rent them for the special occasions of weddings, funerals, and special celebrations. There are a lot fewer (per capita) rental kilts in the US/North America than there are in Scotland, and a lot more kilt wearers who have multiple kilts of various tartans in the US/Canada.

Tough question to really get a hold on. Probably the best way to get any kind of valid answer would be to ask a couple major vendors on both sides of the pond to whom do they sell most of their kilts (including those expected to go out for rentals), North America or Scotland based customers.