UPS Technology - Today there a 2 types, Line Interactive or Online/Double Conversion. Online provides the cleanest power and is generally a lot more money. If the existing spec is Online, its only fair to compare a similar model. But it is often over requested.
VA/Watts - These are the UPS's electrical capacity. The VA is often part of the model number and headline spec. The watts can be 90%, 80% or 70% of that value depending on the units Efficiency. So, if someone asks for a high efficiency 2200va capacity unit, I will probably spec a 3000va, to get their desired watts just because upgrading to a high efficiency unit is more than upgrading the VA.
Voltage - Tthis is where the spec pages can get you in trouble. 120volt is simple, but higher voltages are much more complex. There is big difference between Voltage Range and Nominal Voltage. Ignore the Voltage Range on the spec. UPSes are pre-set to accept 208, 220, 230, and/or 240v. A 208v system wont work on 230v and vice versa. Also, UPSes cant normally change the power. If you want 230v output and have only 208v at your building, you can't buy a 230volt UPS. Nearly every commercial building in the US has 208volt. Rarely would you have 240 or anything else. Look at my last post about how we have managed the different types
Plugs and Ports - All input plugs can be changed to match your needs. The backplate or output receptacles can generally be changed as needed. So the only port that is important, is the XL or Extended Run port. Nearly every unit (except those made by APC) has a port to add extra battery packs to to add runtime. Like the Online spec, its important to make sure customers know they are usually paying 10-25% more for this feature.
As for the remaining specs, most aren't critical and usually match among manufacturers. Frequency, Crest factor, BTus, Environmental, Surge Protection, Wave form, are all about the same for a given UPS size and type.
Two other factors, physical size and runtime, usually can be handled once a particular model has been chosen. Smaller units usually cost more, and runtime is completely dependent on battery size.
