If you were only going to cast the small RBs, then the smaller pot would make sense. ( 10 lbs.) But you say you are also going to cast .60 caliber balls, and those take a lot of lead. I would seriously think about getting a 20 lb. pot. It takes a good sized pot to keep enough lead hot enough, long enough to pour any quantity of large lead balls. 325 grains( the weight of the .600" RB,is 3/4 oz!) 6 balls weigh 4 oz. one pound is only going to give you between 20 and 24 balls. If you use the smaller pot, and fill it only a little more than half way, you can go through 5-6 lbs. in less than an hour making these large balls. YOu will find in casting, that adding new lead regularly is the key to keeping enough lead in the pot at temerature, that you don't have to stop and wait for the lead to reach mold temperature. It is perhaps the most difficult part of casting to learn to do. Any size pot is going to be much more forgiving if you are using only a little weight of lead out of the pot for eat casting. When you start casting the big boys, you are going through a lot of lead fast.
Use the big pot. Check with other makers, to compare prices. . I believe that SAECO still makes lead casting pots.