I was up real late watching EVO, so I figured I'd just join the Splatfest as soon as it opened and get it out of the way. Took three-and-a-half hours to get to Water Slide King, finishing with a Vibe of +19.5, thanks to a couple extended streaks with good teams. The first one had a pair of deadly rollers on it, but I ended up DC'ing on the results screen after a while. The second one wasn't as immediately "lethal", but eventually proved to be very successful. We didn't always have the strongest starts, but we seemed to be able to push through the enemy team near the end of the game (the only time it actually matters) fairly consistently. We only lost a small handful of matches together, two of them because the enemies ended up having three Inkzookas on their team lol. I was at 90/99 when I ended up DC'ing in the middle of a match due to insane lag (I lasted like half a minute without the ink on the ground updating), but thankfully I won both matches after that.
Currently I have no idea which team might be more popular based on actual matches. I didn't run into any Water Slide vs. Water Slide situations, but I also haven't seen anyone talking about the reverse. Then again, it's too early in the day for that to reasonably be happening, so I'll check back later. I did see a fair amount of N-Zap '89s and Octobrushes at least. I also find it amusing that the three stages are Saltspray Rig, Port Mackerel, and Bluefin Depot - i.e., three stages near water, when one team is "Water Slides". :V