If you own the system, you are likely to achieve higher long-term cost savings, but it requires a relatively high initial investment — even after incentives. With leasing, you can start saving from day one with a lower electricity rate than grid power, but your long-term savings will likely be lower than with purchasing.
Leasing also has the advantage that the leasing company is responsible for all repairs over the 20-year life of the lease, and since the lease includes a performance guarantee, it is in their interest to make sure the system is operating as much as possible. However, it should be noted that PV technology has an excellent track record of reliability, and even if you own your system, you can expect many years of trouble-free operation and you are unlikely to have to pay for costly repairs. This is not like owning a car (see more info below on warranties)