That .530 Rounb Ball weighs in a 230 grains, which is a lot of lead. It will kill an Elk very dead all the way out to 100 yds, and beyond, with the proper powder charge. Stick with the round ball. The difference in diameters between the two offerings, and the reference to the " target ball" being .535 is that people shooting paper often want a very tight fitting ball and patch combination, one that has to be pounded into the barrel with a short starter. Fine for the range. Not exactly the best idea in the field, particularly if you are needing a second shot. Obviously you can take your time and load the tight fitting ball in the gun before you begin your hunt each day, but most would suggest at minimum carrying the smaller, .530 diameter balls in a ball block for reloading. If you can, try shooting both in your gun, and see which seems to shoot the most accurate for YOU. Then decide what you want on those occasional big game hunt. Most prefer using the smaller ball for hunting, but that means nothing. What you want to use is the most important. Some barrels prefer one ball size or the other, and that is why I am suggesting to you that the only way to really answer your question is for you to try them both out in YOUR gun.
In choosing patching material, it will help if you use an inside caliper to measure the groove diameter of your barrel. Then measure the bore diameter- the distance across the lands, to determine how deep the rifling is cut in your barrel. The patching is suppose to fill the groove on both sides, and be compressed by the lead ball. So, for example, if the groove diameter is .540, and bore is .530, using the .530 ball and .015 patching will fill the grooves, and also compress the cloth to hold the ball tightly to impart the spin of the rifling to the ball.
Get Dutch Schoultz's Accuracy system- see Member's Services above and go to the links to find his website. It will help you develope a load for your gun with a proper ball and patch combination. He also teaches you how to read your spend patches, so they tell you what is actually going on when your gun fires. A very well spent $15.00.