I have a MyChron 4 and use the speed sensor. If using the Split Time or Time Compare is important then the speed sensor is needed. I found that the GPS data for the M4 drifted too much, even when comparing data from the start and the end of the day. The speed sensor old data (months old) could be overlaid with new data without resorting to shifting the time.
Aim says that the M5 is much better, I don’t know, as I haven’t tried one yet.