Interesting - ran a casual 20 miles last weekend and a marathon yesterday. In both cases, I had Apple Watch 2 on left wrist and Garmin 235 on right. Ran the 20 miles with someone who had my iPhone in their rucksack, so I'm not sure which GPS was used, but looking at battery life, I reckon I must have been close enough to the phone to use its GPS. For the marathon, the phone was turned off and in the baggage area. Anyway, in both cases, the built in running app disagreed with the Garmin by about a kilometre. Yesterday, Garmin had 42.3km and watch had 43.43 and it translated to a Garmin pace of 5:23 and a watch pace of 5:15.
I had been (ambitious last minute decision) aiming for a 3:30, convenient 5:00km pace which I maintained for a lot of the way. If I had been serious about getting to the 3:30 time and solely reliant on the watch, I would have missed by 2 or 3 minutes.
I'll use my apple watch for regular, casual runs. But, I'm thinking that for events or anything where I'm going to be sticking to a pace, I'll keep the Garmin.