I have tried both styles, and enjoy the u-shape best. I got mine new for about $200 at the old Sportsman's Warehouse in Salem. It lets you sit with your body out of the water, has better manueverability, and has a nice storage space right behind the seat for tackle box, anchor, net, etc.
You are more than welcome to take mine out for a spin to test one before you buy. Just drop me a PM and we can set it up. I do agree with Irishrover- if you can afford a pontoon, and have the means of transport, you should look into one. I went with the tube because I can throw it into the trunk of my Taurus and go, leaving the gas guzzling Suburban at home.