All of them should provide enough power to shut the system down in the event of a power failure. The real determining factor for need is the amount of blackout/brownouts you have in your area and what is running on the PC.
If you have good power, and rarely have blackout/brownouts - it may not be needed.
If you are running a server (web or local) that has a lot of read/writes to the system, and a blackout/brownout would create issues, then it would be a good idea to have one. In most workstations, the worst thing that typically happens is you lose the spreadsheet/word document that you are editing when the blackout/brownout occurs.
It all depends on the power quality you have. Just from personal experience for the time I have lived where I am at I have never had a UPS until I just recently purchased one a month ago I had the money to burn and have always just wanted one, using the same PSU and a few of my hard drives are several years old with no UPS being used prior with no issues and this is living in Florida. Although during summer you can except thunderstorms pretty much late afternoon like clock work - it would be frustrating playing and having the power cut out. Only on a few rare occasions has the power been off for long periods of time in which case a UPS would really be pretty much useless in regards to powering something big. The cool thing is that if I ever experience a extended power outage it has two USB ports so I can charge my cell phone several times or my flashlight too.
That being said I did allot of research on them before I ended up purchasing one. I decided to go with a CyberPower CP1500PFCLCD PFC compatible 1500va 900w UPS
- http://www.amazon.com/CyberPower-CP1500PFCLCD-Compatibl... - It's PFC comptabile which is important due to most modern PC's won't run on one unless it is, the LCD screen that displays information is nifty and useful, the software that's linked to the UPS is pretty good so far and it's actually a pretty small unit not to mention it was pretty affordable compared to other PFC compatible units on the market. I tested it out one day and ran the system in my sig along with a 27 inch and 22inch secondary monitor, cable modem during idle for about 5 minutes with 35 minutes to spare according to the software