You should probably heed the advice on the more experienced paddlers by going with the bigger boat. But I recently spent 5 days on class 3+/4 in the 8.1 (the newer creek version) and absolutely loved it. At 205 lbs+ there was no issue whatsoever with it sitting too low in the water. At 6'3 the seat and footrests were adjusted back as far as they could go; with booties on, my heels (size 10 feet) were touching each other and my toes grazing the top, but I still found it very comfortable.
As far as paddling performance, I found it superior to my large bazooka (the only other creek boat I have ever sat it), in every aspect except maybe stability.
I found the mamba easier to roll: its the first time I felt comfortable being upside down in the rapid, almost knowing I was going to come back up; rather than oh shit, I hope I can pull this one off. I know that has more to do with the mental aspect than the boat; and a good paddler should be able to roll any boat; but I am curious has anyone else found the bazooka hard to roll? I find my XP10 is easier to roll than the bazooka and my first time rolling a playboat, I did a 360 (upside down again). Sorry to get off the subject...