I don't have any experience with a 96" tube, but I do have the O'Brien Ultra Screamer 80" tube. It's comfortable for three riders and pulls very easily. I guess if you are looking to pull four riders on a single tube I might go for the 96".
I guess it comes down to what you want to do with it. If you want something large enough for a few kids to play on or a few people to lay out on while anchored, then you may want the 96". Storage may be an issue. If you blow the tubes up before you hit the water and carry them inflated in/on the boat, then smaller would be better. If you blow them up and deflate for storage while out on the water then not a big deal.
From my experience riding the 80" by myself (6', 190 lbs.), I think the 96" would be too large for a single rider.
BTW...you may want to look at the Sportstuff Booster Ball Tow Rope...it helps keep the tow rope from dragging or getting caught in the wake of the boat when crossing the wake with the tube...it also knocks down some of the spray that the jet pump produces (although this may not be as big of an issue with your twin engine setup?).
Either way you go you should be happy!