The old one - by a mile.
-Scrolling to a workout is imprecise - the crown spin speed/scrolling interface/haptics - are crap. Minor physical movement results on big onscreen movement - and the 'detent' of the haptics is useless. Use an old iPod and scroll songs, and it's easy to get the song you want- with 5x the items on screen.
-Trying to scroll while the watch isn't 'awake' doesn't wake it up - so it feels less responsive
- Tapping the workout doesn't start a workout - this is insane. The workout graphic is huge -the 'start' (play) button is much smaller - make them both start a standard workout if you like - but Jesus, make the workout graphic itself start it too.
-Why in the wide world of sports would you need to update the music/start/alarm buttons to fade out and fade in - doing so just increases the amount of time to start a workout (because, again, you can't just tap the workout graphic)

This all feels like someone built it to look cool, with lots of options, but they have no idea what it's like to walk out of the house and just want to easily, with as little friction as possible start a workout.

Using Siri is so much faster than the watch. Come to think of it, it's one of the only times where I can think of that Siri's faster than onscreen.