No matter which tube you get, spend a few extra $ and get either a robe with bungee cord in it, or get the Booster Ball. The bungee cord helps take some of the shock out of those launches when the rope is quickly pulled tight, such as when cornering and the rope goes a little slack. The Booster Ball helps keep the rope out of the water to reduce drag and put less water in the tubers faces.
If you have younger tube riders, they might appreciate something that they sit in, and something with a backrest so they don't feel like they are going to fall off the back. Also, you absolutely want a tube that has an inflatable bottom--that is, the part where the rider sits needs to be inflated, or they will take a beating from the water.
I shopped online and got a great deal on my tube from boatersworld.com (though, as a 3-person model, I wouldn't recommend it for you). I purchased a tube made by Rave Sports, and it's well made with heavy-duty vinyl and a cover that covers the bottom and all sides. Some of the online sites like ravesports.com have videos of the tubes in action to give you an idea how they ride.
Of the "kits" I saw, they were tempting, priced well, but I found they had only the basic ropes and low-end pumps that looked like they wouldn't be very durable. Maybe someone else has some actual experience with those.