I've stayed in both towers, and I don't really see a big advantage to either. Since you are island view you might want to request the Waianae tower (the one on the right as you look toward the ocean) as you possibly might be looking toward the mountains north of Aulani along the coast. For you, I think my biggest suggestion would be to request a high room.
For folks with little kids, they might want the Ewa tower as Auntie's Beach House is there. Otherwise I think both towers are awesome! The spa is in the Waianae tower. Happy planning!
I much prefer the Waianae Tower - it just seems more compact and less walking. We pretty much have gotten the Ewa Tower for our last 4 visits and we seem to always be right down the end on top of Aunty's House - which can be a pain if you need to make a quick dash back to the room for something.