Some smartwatches have a touchscreen interface, although most work with an array of buttons. Touchscreens can be very intuitive and easy to use off the bike, but when you're riding they can be awkward to operate, so having buttons as well is useful as a fallback.
The decision between touchscreen and buttons is more personal preference than anything else. Buttons can have a more tactile feel, are much easier to use when wearing gloves and guarantee impressions, whereas the quality of touchscreens, and their performance in wet weather, can vary from brand to brand.
On the other hand, touchscreen displays are often more aesthetically pleasing with less bezel and sometimes with smaller screens. Usually, a touchscreen smartwatch will offer buttons too. It may only allow you to use the buttons to control functions during exercise to prevent you from inadvertently stopping recording as you ride.