There are two major differences between the S7 timers and the IEC timers:
1) The maximum amount of time that you can use "S7" or "SIMATIC" timers is 2 hours and 46 minutes. If you need to time more than that you can make use of IEC timers or add counters to the S7 timers (build your own custom timer). Here is a video that shows you how to do that:
Extended Timing in PLC Programs Using Comparator Operations - Part 1 https://realpars.vhx.tv/videos/extended-timing-in-plc-programs-using-comparator-operations-part-1
2) IEC timers consume more memory space. So if memory is a concern for your project, you may want to go with the S7 timers instead.