I am no expert on this either but I believe it has a big factor or merchandise and a medium factor of attendance. I really am guessing based on just one example I know of, a bad team who should have poor attendance but are 100% national players and have really good attendance.
I think if you can get a similar player for a similar price always go for the national. But if you have to pay a lot more or get a weaker player then always just get the best player available. This is easy for USA, harder for Australia, impossible for Barbados!