I don't know about any moisture build up in a smoker, usless you are using a water pan and cooking at a high heat causing a lot of steam, but I have only used small backyard smokers. I have made, designed, modded, quite a few backyard smokers and have not had a problem with anything dripping from the lid onto the meat.
My understanding is that the round top will aid in circulating the heat and smoke better. With that said, many of the Brethren use flat top lids on their UDS's and many large manufacturers make box shaped smokers like vaults.
I guess it is up to you and your preference and what is easiest for you to manufacture and what materials you have available.