In terms of overall efficiency relative to where you can get the other stats, the Great Wheel is ahead as attack % is significantly more space-expensive than defense %. However attack % is also generally more common than defense % - sentinel outpost being the only *great* defense % building.
I'll be taking the Wonder Wheel on live because of part of an overall plan where I think I'll have enough attack % within a year or so, but defense % is still something I want to work on. It's also a better BG collection.
I'd say they presented two good choices on the building this time though where you can't really go wrong either way