There are choices to make when looking a pontoon boat or a cat boat like steel or aluminum and even stainless. First with aluminum in order to get the same strength that you would get out of steel you would have to use schedule 40 structural aluminum.
Most pontoon boats made today use drawn aluminun tube which is good for mild river and lake use drawn tube is not as strong as aluminum pipe but remember with aluminm moving parts oblong out. Aluiminum pipe fames weigh as much as steel frames. Some of the larger cat boats use 1 1/4 to 1 1/2" aluminum pipe.

With steel you can use a thinner wall and smaller diamater tube to get the same strength. Down fall is rust allthough galvinized tube works real well where ever there is a weld the galvinze is burned off. Powdercoating can only be applied to the outside of the frame. Some manufactures go the cheaper route and spray paint the weld area.

As far as pontoons go you will here why everybody tubes are better than the next these are the things you will have to sort out on your own. Call all the manufactures and talk to someone and go kick a few tires.

Buying a pontoon boat can be very difficult there are so many options to choose from wether it be a Outcast, Bucks Bags, Water Otter, Skookum, BlackBear, Waterskeeter to name a few.

These are some food for thought 15 years as a certified welder has taught me a few things about metal and fabrication.