Wilderness Systems I find has by far the most comfortable seats.
As for the "filling up with water" risk of a sit-in, this is why you have to carry a pump. Anything longer than 10' usually has separate compartments so it won't sink. Practice getting in an out though.
I am with WhiteH20.. sit-in for sure. 12-14 foot range will probably make you happy. Used roto-molded is what your budget dictates.