Hi Kswan,
There are good reasons to use wax cast parts and there are good reasons to use sand cast. Reeves Goehring's castings are very good sand castings based on originals. With a fine sand or clay, the cast part can be very smooth. Wax castings are fine if you can get the design you want and you don't intend to modify it very much. Many of the sand cast parts have a lot of extra metal, which can be very useful if you intend to modify it very much. Wax casts are often very thin in spots making reshaping them risky. I found this stuff out the hard way. Today, I buy Reeve's castings if he has what I want, wax castings if they suit the job and sand castings if they are best to meet my objectives. As a last resort, I cast my own. I sometimes have to do that for pistols and 17th century guns.
dave