If you're happy with the S100, then no need to upgrade. The primary benefits of the S300 are real-time programming, datalogging, and programmable outputs. It's better suited to someone that will tinker with the tune on a regular basis or needs custom outputs, for things like PWM boost control or butterfly valves in the exhaust, etc.
One reason you see more S300 setups today is because Hondata stopped selling the older S100/200 stuff. Also tuners expect you to have a modern setup, so you may in fact need to upgrade if you can't find a tuner willing to burn you a chip for your S100 ECU.