Is Apple Watch Series 2 or Fitbit Flex 2 better for swim tracking?

I recently left the gym in San Francisco and ran into my friend Bob. Bob has competed in (and won) international swimming championships, and he was on his way to swim laps. I told him I'd just done the same thing, in an effort to test the new Apple Watch Nike+ swim tracking features. (Apple Watch Nike+ is the functional equivalent of all Apple Watch Series 2 smartwatches, but it has a proprietary Nike sports band and some different clock faces.)

Using the iOS Workout app on my iPhone, I showed Bob the data the Apple Watch Workout app recorded during my 46-minute swim, including active and total calories burned, total distance in yards, average heart rate, number of laps, and average pace. I showed him my average pace per 100 yards, too. And I showed Bob how the Workout app on the Watch lets you choose three different criteria to focus on during swim exercise: active calories, elapsed time, or total...