Boats called river runners usually feature a planing hull with slightly less volume and better edges than the large round displacement hulls used in most boats called creekers. This makes for a sharper ride/better steering as long as you're in control, but they might prove a little edgier and easier to get grapped or trown of course. One of the reason that for really big creeks a creek boat is adviced lies in the point of the boat. A bow with a flat point is easier to get stuck under a rock or in the riverbed. It doesn't happen a lot, but it can lead to nasty situations. Overall a river runner might be better for your technique, and will serve you fine on class 3 and 4 white water, but on class 5/5+ water a creeker might be better for your health. However, as you apparantly are not an experienced white water runner, I wouldn't advice you to paddle class 5 right away in any boat.