A UPS is designed to uphold your load upon a mains fail for a pre set length of time. Therefore, depending on the criticality of your load depends on which type of UPS you should order.
If your load is critical and any dips or spikes in the supply would cause problems with your load then Harland Simon recommends that you order a true online UPS.
If your load is somewhat sensitive to peaks and dips in the mains then you can opt for line interactive with the added buck or boost feature.
If your requirement is non-critical then you can opt for the offline UPS.
All of the above somewhat differ in cost and Harland Simon recommends that you spend accordingly with regards to how critical your load is.