I have a Garmin sport watch with a touch screen (plus two physical buttons). If you're out in the rain and need to change modes on a run or bike ride, it's nearly impossible, you need to stop, bend over to shield the watch with your body, dry your finger on your shirt (assuming you have a dry spot), then then you can usually use the touchscreen.
Once it's set to the right mode, the 2 physical buttons work well, but anything that requires the touchscreen is nearly impossible without stopping what you're doing.
(at least that's the case with whatever touchscreen technology Garmin uses, maybe there's a better option for use in the rain)