Really torn between Colosseum
For medals or Aachen for coins.
Neither!
As sources of MEdals and Coins both are awful, there are many better options.
There is NEVER a good reason to build Colonobuildem. Just don't build it. You want Medals? Find someone power leveling an Arc and add FP to that Arc. You should be able to net a minimum of 10 Medals per FP and if you are disciplined a lot more.
If you fight CoA is a must have, But if you do not fight, don't build Aachen, it's a lousy Coin source. Instead in the space build 6 Apartment Houses and a Cultural Building to keep them Enthusiastic. If you keep them motivated, you'll get 12K Coins a day from them equal to the output of a lvl 8 CoA. And you can spend the FP acquiring BPs (as wekk as Medals) for GBs you do need or want.
If you need a GB to help with Coins, build SMB. It's 50% bigger then Aachen but you get a lot more bang for your buck. Goods and doubling all your non GB Coins collections.
Again, if you do fight, build Aachen, if not, don't build either.
Good luck!