Snoce pretty much summed it up, and with your 2nd choice (30 gallon tank: 36" x 18" x 12") you have a much larger footprint of 648 in^2 as opposed to only 360 in^2 for the 29 gallon. Fish often like to "stake out" territory, and having almost twice as much real estate makes for happier fish, or more fish. Either way it's better.