I'll go with Atomium once I''m on AA.
by lvl 1 upgrade you save 2.4 hrs
By lvl 2 - 4.8 hr
by lvl 3 - 7.2
by lvl 4 - 9.6
by lvl 5 - 12 hrs
by lvl 6 - 14.4 hrs
lvl 7 - 16.8 hrs
lvl 8 - 19.6 hrs
lvl 9 - 21.6 hrs
lvl 10 - 1 day
Add that up ,an you can do all lvls of 1 upgrade 129,8 hrs faster which is 5,4 days faster.
That's really something . Considering there are 50 skills at Uni , and I have around 10 of them researched and not maxed I still must do 40 of them .
40 times 5.4 is 216 days that I save on Uni researches and can use the saved time somewhere else.
I can do no Uni researches for 216 days and concentrate on other upgrades in that time , and end all Uni researches with same ammount of time than someone starting them 216 days faster.
That's really huge advantage.
Given , that Uni researches of f.e. Mortar benefit all mortars ,not one like it is when you upgrade mortar directly, and 4 lvls of investment in dps or health of thise give you more in return than upgrading mortar from GPA to GBA , and that Uni researches are cheaper , and take less time than upgrade of one mortar from GPA to GBA I'll concentrate on Uni researches rather than on upgrading defenses directly.
Atomium will make me able to be more piwerfull on defense even faster .
Really in my opinion there's no AA wonder that is more beneficial to you from economical point of view
Sydney Opera House provides 1 additional citizen , and lets you do thanks to one more citizen upgrade one defense more, since after barracks and ofensive upgrades will be done that's where you'll send yur workers to be busy, but it is really more payable to do upgrades at Uni first , and then do defense upgrades directly.
That's why in my opinion Atomium >>>>Opera House