<< Does it really matter? it's either tenure or no tenure. >>
Assistant = no tenure. One goes up for tenure usually 5 years after hire and if they have enough criteria met (there is a LOT usually) they become tenured and move up to Associate. Then, years later, they can go through a similar process and be promoted to full Professor. If the Assistant fails to get tenure, they typically leave within that year and move to another school or job to start again.
An Assiatant Prof typically starts between 35k (low end) and 80k (high end), depending on school, department, Dean, and how well they negotiate. Average is probably mid 40's