It's irrelevant how many answers I have provided thus far, I haven't been on this community much yet that's all. But I have worked in the industry on machines (electrical, electronic, IT) installing and maintaining for 28 years, have every professional qualification, I hold a UK FIET membership etc. etc. But we need to keep this in layman's terms.
Modern computer power supplies are more efficient than ever, and also incorporate something called "active PFC". Active PFC is a good thing, and more and more power supplies are using it, including iMacs.
Active PFC and stepped sine waves do NOT always play well together. If you get a new computer, or a new power supply, it's possible it might not work with your old UPS. If you buy a new UPS, you should make sure it produces a true sine wave output, not only because you're current computer might need that, but because your next computer is even more likely to require it.
There are differences in quality of UPSs and other variances but for the relatively small extra outlay I highly recommend pure sine wave UPSs and stand by that for the OP question.