JK boats have different charecteristics based on redesigns every couple years... the 2011 and newer superhero is an awesome, comfortable and very stable boat. it also rolls very reliably. although they are somewhat slow downriver....
a >2011 would be a major upgrade to a 2008, and cheap if you can find one. I have done the best boating of my life in the 2011 superhero. some creeking, mostly river running, even a couple waterfalls on brush creek, even tho i am primarily a class III/Iv boater.
for a newer boat You should check out a 2015 Zen LG for river running class III/IV. Villain and Karma are as Dirtbag described, designed for creeking, big drops, class V etc. although they will do fine on class III rivers too, they are larger boats (1' longer bow on karma then superhero)
one last thing, which turned out to be a massive fail for me, was that they eliminated the high knee bumps on the older boats with the new karma and rockstar. (smoothed out the hull in front of the cockpit.)
in comparison, for my legs, i have all day comfort in the 2011 SH, 2012-15 zen, and 2012 superstar.
I demo'ed a Karma M and my feet were asleep, like the old days, (like afraid to stand up no feeling asleep) in @ one mile. that really, really sucks, because other then rolling reliablity and performance, comfort is one of the biggest reasons to paddle JK.
user experience may vary... but thats what happened to me. to be really thorough, I should have demo'ed a Karma LG, but at 105 gallons, it is a Supertanker of a boat. seemed too big for me. A 2015 Zen does work for me and is my new boat as of last year.
hope this helps a bit.