i always like cooked pudding better, but there is so much sweet stuff in a dirt cake instant pudding would pass. My husband used to make this for Christmas potlucks at work. He would use a new plastic flowerpot ant place a plastic poinsettia in it.
As a rule of thumb, homemade should taste better than store bought, but sometimes that's not really the issue! For Dirt Cake I'd probably use a mix and just enjoy the end result. Sounds like you're going to have fun making this - good luck!