Well, size tire and load are doing two different things. C load are right for the weight of your TJ. The E load will be way to stiff and make for a rough ride.
A smaller tire will give more onroad performance due to them being closer to ideal gearing. A larger tire will give better offroad performance due to ease of rolling over objects. I don't know what your'e trying to accomplish overall, but if you get a 15" wheel you can find lots of 31", 32" and 33" tires with a load range C, and a few 35's. Any of those would be good combinations.