My son and I are going to buy new float tubes.

It looks like the Fish Cat is the best bet for him, at 5'11" and 190. I am 5'9" and 250, is the fish cat big enough for me or do I need to go for the fat cat? Cabelas has a Kennebec that looks good too.

I don't mind sitting down in the water more, this is a way to be able to get out and fish when it's hot out.

I have tried several different tubes and the fishcat has been the best for the money. I am 6' 1" and weight about 270 and have no problems with the fish cat. The only complaint I have is there is no front bar to keep the float pontoons apart when i fill the seat but most of the guys like the way the net lays across without the bar. Prob a results of the weight limit causing it to come in some. Seriously. *Knock on wood* I have seen hundreds of snakes and never had one come to me other than chasing a top water. One small splash and they run away faster than I ever could. I only fear the snake on the entry and exit at the shoreline.

I do not have a lot of experience yet with my float tubes, but I bought a Fish Cat 4 and then found a deal on a Togiac. The difference being the bar - Togiac has one the fish cat is only the stripping apron.
Both are easy to maneuver and cast out of - but I have only used then a handful of times. I am 5'11 and just under 225, my 15 year old son is about 5'9" and about 150lbs. He prefers the Fish Cat.
It took me a while the first time to get the hang of controlling my position with fins but I think I got it down now. I think you would be happy with either one.

I have had the original U-BOAT for more than 10 years... It is a great fishing platform and has plenty of storage. I love the stretch across mesh work deck, and it is really easy to get in and out of. The Original U-Boat is still available at Cabela's and the price is unbelievable! I think you can pick one up for around $69.99 these days. Back when I got mine, they were $129.99... It has a moderately high back, but never got in the way of my flies or any other tackle. As a former fly casting/fishing teacher, I recommend modifying your casts anyway as an added precaution whenever deep wading/in a float tube.

I should say that I have owned round float tubes and done quite a bit of fishing from them in the past. The last ones I had (I bought 3, for me and my 2 sons) were made by Browning, and after having the family jewels crushed every time I fished in one, I swore my next tube would be a top of the line model.

U-tubes are less maneuverable, but much more convenient in terms of getting in/out. They also generally have more storage, and they are much more comfortable. As for brands, go with Outcast (makers of the Fishcat/Fatcat line) or Buck's Bags. Both are very durable and have excellent warranties.

I have to hike about 3/4 of a mile to my favorite spot. Is the fish cat as easy to transport as the roundies?

No real difference. You can get you some backpack straps and, with a little creativity, create a nice carrying system, too.