You will want the wedding cake to be the main, larger cake. The groom's cake is the smaller, "fun" cake. Usually, I will design the wedding cake first and figure out how many people it will serve. This will tell me how many servings I will need for the groom's cake and then I size the groom's cake accordingly.
For example, you said that you need to serve 150--you could do a 6", 10" and 14" wedding cake (serves 124) and then a 9" groom's cake (serves 30). There's are lots of ways that you can do this--that's why I start with the design of the wedding cake, figure out how many servings I will need for the groom's cake and then go from there. HTH!