Volume is the key. We lease quite a few machines from HP, and get much better rates (less than 17%, above 12%) per annum. When we started, we were getting MUCH higher rates on lease. A friend I have who works at Nationwide Insurance pays about 40% of the list price for servers from IBM.
I find that if you get leasing through a 3rd party leasing company not related to any of the computer companies (HP,DELL, etc...) that you can get very good deals. We lease some servers in the 12-15% range through them. You just take the quote to them, they buy the equipment and than lease it to u. Real simple... Real cheap...
You will almost always pay more interest on a lease than most other types of credit. With a lease, the only collateral they have is equipment that depreciates quite quickly. This equipment is also moveable, harder to track, etc. increasing overall risk as well. In a mortgage, real estate prices almost always appreciate, etc. greatly reducing the risk and student loans are generally backed by the US governemnt in some way, again reducing their risk.
If you have credit lines other than the lease that have lower interest than by all means, use them first. Generally working a general business loan will also garner a much lower interest rate.