I was skeptical with Splat Zones at first, but then I realized how intense they could get. I think I prefer Turf War overall, it's a little less hectic and a little more strategic, while Splat Zones are more competitive and twitch -- relying on good players and teamwork.
Both are really good competitive modes, though. I mean, I know Ranked is supposed to be the "competitive mode", but Turf Wars can get really crazy if you have two good teams go against each other. I think Splat Zone feels more intense because of the victory conditions, instead of the timer and comparison of Turf Wars.
Anyway, I'm interested to see what Rainmaker is. My only gripe with the game is I wish we had voice chat. Other than that, it's fantastic.