Re: What Kinda Tube Do you use??
I can see where two people on the tube can add to more injuries. With the Bonzi, the boat driver is really in control. The driver has to "work" at losing the riders. On tight cornering, when tube spins out to the sides, it does not catch and flip like flat tubes. If you have real agressive riders they go by themselves.
Most of our riders are teens and early tweenties. The girls like going together, and the wave inticipation screams add to the fun. I think it becomes, who can scream the loudest. They also like to face each other, leaning against the 45 angle, and gab while we put around the lake. The boys like to show off, and go by themselves. Many times they like to see how long they can stand-up or from kneeling position. If the driver finds or creates the right wave, you can get some good air too.
Our single rider tube only sees action at the beach.
In two seasons the only injuries we've had are fatigued arms and a little tube rash. We are usually in the 25-30mph range, which is fast for tubing. I think the recommended safe is 15-20.

We use a 4-person tube rope. After 2 seasons, no cracks by tow-ring, but plan to re-inforce it this spring.
The price on the Bonzi was around $125. I was skeptical it was the same quality and construction as the Rampage. An email from Sportsstuff confirmed the only difference was the graphic. I got mine off ebay, but hear that Costco sells them, too. We are on a small lake with a single store that sells O'Brian tubes. As of last season, there were three Bonzis on the lake. The year before, we were the only one.

I bet there will be a few more this season.
Good luck with your choice. No matter which you choose, you will have fun.