Go to "view & buy" click "view" next to the planes, and you should see a sales / lease link box on the right if you own that plane and it doesn't have any route on it.
Owning / leasing depends on the rates / price. It also depends on the return you could get on the plane. You have to decide for yourself. Paying $$ every month is not absolutely worse than buying, because you don't need to pay the whole chuck up front. Instead of buying one, you might be able to lease 5 or more. However, it is not easy maths to figure out which is better, and you have to do it on your own.
Lease are paid monthly, at the time you take the lease and eveyr 1st oft he month thereafter. The rate is the monthly rate regardless of period.