Inside sales is answering questions for customers who are already interested in purchasing a product.
Outside sales is canvassing and cold-calling to get someone to purchase a product.

Which is harder? Which should be paid more?

Outside sales is much harder and IS paid more. This is not even a close contest when you consider the time and effort involved to achieve the desired goals of each position. Inside sales people get to see their families and sleep in their own bed every night.
Having said that, inside and outside sales departments usually work as a team.