There are a lot of things to think about when you are in the market to procure a mainframe processor. Business requirements are paramount, but technical requirements are just as important.
Are new generation features needed?
Are your channels buried and you really need those 16Gbps channels? Would terabytes of memory or SSD technology really improve your performance? How about faster IFL or zIIP engines? If none of these things or other high tech improvements are top ten on your Hit-Parade, you may want to reconsider that new z13 purchase and consider the lower cost option of a used processor.
Are MIPS scalability required?
Do you need the ability to dial your MIPS up and down on demand? Are you using Capacity on Demand? Well, there is a pretty good argument for saying on a processor that is still under marketing support. You can kiss these features goodbye a year after your processor is withdrawn from marketing. On the other hand, if dynamic scaling is not needed, but you want to be able to increase or decrease MIPS as applications are conceived or retired, a used n-1 or n-2 might not be a bad idea. In many cases, buying a used n-2 CPU with more MIPS and doing a box swap can work out cheaper than doing an MES upgrade on an n or n-1 box. If you can tolerate the outage window for a box swap, upgrading an existing processor in place is not always the most cost effective option.
What are the compliance requirements with other gear in the enterprise?
Are you running in a parallel Sysplex with processors of various generations? All of the participating processors need to be within 3 generation window. This can be pro or a con for a new processor depending on your situation. If you are in adding a processor to a Sysplex that includes a z9 you can forget about that new z13.
There are a few other financial considerations to think about too.
Make sure you consider the one year warranty period on a new processor during which you won’t need to pay for a maintenance contract in your cost analysis of new vs. used. If you are leasing a processor as part of a package deal that includes your IBM software, then that changes things too. When purchasing a new processor that is larger and of a newer generation than currently in use, IBM offers a Migration Offering which essentially provides you with a pretty attractive trade in value for your current processor.
This really just scratches the surface of things to think about when in the market to purchase a new CPU, but it brings up some of the decision points for new vs. used. I have used both options in the past depending on the situation with good results. What works for you?