Is there actually a need to do a shutdown, why not just sleep?
You buy a UPS for 1 reason only: prevent data loss due to loss of power. In order to prevent data loss you need to shutdown the machine or use suspend to disk (you don't need to power the machine in that case). However, in some cases this can be a bit problematic since not everything comes up correctly when resuming after being suspended (as we've seen with virtual machines). For those machines the only real option is to shut it down completely.
If you use the built-in UPS support in OS X you don't have much of a choice though since you can only choose between shutdown and shutdown. If you use something like apcupsd however you can create your own scripts and run those when something happens. This way you can shutdown the machine or use the deep sleep (suspend to disk) option. In case of vm's it's best to shutdown the hypervisor and let the hypervisor decide what to do with the vm's. In some cases it means a shutdown of the vm, in other cases a suspend.
Both shutting down and suspending requires time and that's why it is very essential to take this into account when setting up when to shutdown/suspend. You want the machine to have completed the shutdown/suspend action before your UPS runs out of power. In some cases there is even an option to shutdown the UPS as well. Biggest issue with this as well as with computer shutting down: what to do when the power is restored and the machines can be booted up/resumed?
Which correlates with what nanofrog was saying, and I quote:
No it does not as your quote is clearly stating that. Your quote is about another form: square wave inverter. It is not about stepped sine wave that is discussed here. The HP file has this to say about what to use:
For the best protection during unexpected utility outages, use a UPS that has a true sine wave inverter. A line-interactive UPS with a stepped sine inverter may be satisfactory if the dead zone is less than 2 ms.
Note the second sentence!
Whether or not the Mac Pro is actually effected by a stepped sine wave is somewhat irrelevant, the equipment was designed to run off a pure sine wave and any UPS that outputs otherwise is simply a cheap unit.
As you can clearly see from the HP document it doesn't matter if you use pure or stepped sine wave. They both are fine to use. Mind you, this document is about what UPS to use for HP SERVER (yes, SERVER!) products. You know, those machines that are one step higher on the "professional" ladder than workstations such as the Mac Pro.
There is a much simpler approach to the UPS products from APC: their SmartUPS line is their more professional line. They didn't aim for a cheap UPS for the consumer in that range and thus did not cut any prices by leaving out functionality or by limiting certain functions except for their SC models (they are a simplified version that uses square wave). It also means that it will have the best support there is, meaning that it is far more likely that it will simply work (work with OS X, Linux via NUT or apcupsd, etc.). If you want something proper from APC then this is the product line one should look at.
What's the difference between the SMT and SMC lines from APC?
They put out new products and with that also changed the model numbering apparently. SMT means the standard model, SMX the extended run ones (previous they put XL at the end) and SMC is the simplified version. I've managed to found something in the APC faq:
Frequently Asked Questions for the new SMX and SMT series of Smart-UPS products.. For the SMC see the following:
Smart-UPS C Series FAQ. To answer your question: they look very similar. You can look for them on apc.com and compare them to see some of the differences.