The UPS depends greatly on what protection your enterprise needs for given applications.  The UPS configuration may already be set, such as static or rotary.  The minimum about of ride-through time can set a number of expectations for some quick decisions about the type and size of the UPS.  If the UPS only needs to provide for a safe shutdown, simple inexpensive UPS systems should be able to perform just fine.  For more mission critical facilities where uptime is paramount a parallel system utilizing redundant systems, dual bus, or combinations may be required. 

There are additional uses that are considered, so the UPS system should be planned to match the data center use or could be modular to provide support the future growth without disrupting the existing systems.  It is also important to enlist an engineering team that understands the current and future needs so that the entire infrastructure will work with the expected IT, and even consider a good amount of flexibility. 

Maintenance contracts and local support will also be critical factors when choosing a specific UPS.  Often this starts with a 30 minute callback and no longer than 4 hours to have someone on site to assist with the equipment.