DS1 sat to wee at first, but would only sit on a toilet, not a potty, I think because we use toilets and he wanted to be the same. He started standing to wee when the other boys at day care were standing to wee.

DS2 also sat to wee at first but not for long. As soon as he was tall enough (and probably even before he was tall enough), he peed standing up, to be like his brother.

There's a lot to be said for imitation.

Now if anyone has the secret to getting a 6yo and 8yo to always get the wee IN the toilet, please let me know.