We have stayed at both.
Here's the thing. The single best rooms/views in the entire resort are Lake Views, if you get the room numbers that end in low-even-numbers -- i.e. XX02, XX04 etc. Those views are spectacular -- you get AMAZING lake views, no views of any other structures or rooms and ALSO get great theme park views. So best of both worlds.
However, the exact same view category might get you odd-numbered rooms on the inside of the structural C of the tower where you have a lake view competing with views of other rooms in the tower. So it's luck of the draw. I have always wondered why they don't have a premium lake/theme park view for those best rooms so someone willing to pay to get them and not take their chances with the odd-numbered rooms can. But they don't.
So if you want to guarantee a cool view, you need to go for the theme park view. You will see a lot of parking lot but you will be able to watch the fireworks every night from your bed or balcony. If you are feeling lucky and want the best view in the resort, get the lake view and request the low-even-numbered rooms. Or if you don't mind looking at other rooms in the Tower and like the water view, get the lake view and save a lot of points/money.