I have sat in the lower end-zone - a few rows from the field, and the upper endzone deck, a few rows back from the ledge. Both are good, but the UPPER is better if you want a good view of plays develop and any hope of seeing what's happening at the OTHER END - when play is down there.
If all the action takes place inside the 30 on your side the lower end zone is a good place to get a feel for the game - because you can hear what's going on pretty well, and see it great. If the majority of action is on the OTHER SIDE tho, you really need to be listening to an ear-piece radio and watching the jumbotron to know what's happening.
I have only sat between the 20's a few times. Those are great seats, too. I would prefer them if all things were equal. You may not get the perspective you get from the end zones, but you get to see action on both sides of the field better. It's more like your TV view.