The two configurations will give you a different look - it will depend on what you’re after and how you’re going to finish it (lots of fresh flowers or just one or two perfect blooms - the lush look will be better suited to the first size below; the one or two perfect flowers would look better on the second size.).
I would do a 15, 12, 9, 6 for something that is proportional - I calculate that this will serve 150.
The 12, 10, 8, 6 is a more columnar look, just as tall as the first, but it will look more “sleek” because it is narrower. This would serve 100 in my book.
I have a thing about fresh cake, so I give my clients a First Anniversary cake when they give me a professional photo of their cake; I encourage them to serve the entire thing at the wedding reception. I also include a note to the function manager that explains this (as well as how many servings they should get, what the flavor is, the client’s name, any special information like whether they are saving the sugar flowers or not, removing the ribbon before serving, etc…)
As for the texture of the chocolate cake, if you didn’t weigh or measure the flour accurately, this would give you the very dense texture - you might also have had the oven temp too low or undermixed the cake batter.