Both of these power meters measure power and cadence, however the way power is measured is much different.
The XC100 is a single-sided pedal system. This means there is only a power sensor in the left pedal. The left pedal therefore reads power from your left leg and doubles it, in an attempt to estimate your total power. This ‘doubling of the left leg’ is how all left-side power meters work.
The XC200 has a power sensor in both pedals. Therefore, it does not need to make any estimations. Rather, it can accurately read the power you are producing from both legs and it simply adds them together to get your total power. This can make the XC200 a more accurate power meter.
In addition to accurately measuring your total power (by adding both legs together), it can also tell you how much power the left is making vs. your right leg. This is called left vs. right leg power, or independent left vs. right power.
The tradeoff here is price. While the benefits to the XC200 are clear, it’s a fair bit more expensive than the more affordably priced XC100. The choice is yours!
Garmin Rally XC Comparison Table