You will find that once you get used to the shape of a maul, you will prefer them to any other striking tool. That said, a 20 oz. may be a little too heavy for carving and tooling work on smaller item where the strike force required is not as great. The 20 oz. is ideal for tooling larger areas such as a saddle skirt or where you want a really deep impression. I have a 12 oz., 25 oz. and 40 oz. on my bench and use them all for different tasks. The 12 oz. sees the most use for carving and stamping work on the sheaths and smaller items and when working with the lighter weights of leather. It is harder to "lighten up" with a heavier maul and you will tire more easily using the heavier weights.
I prefer the straight round, but that is probably because that's all I've ever used. The tapered head seems to be more prone to mis-strikes, but that's not based on fact, just opinion.
Paul