How much of this game is going to occur in Eidolon City and how much in the real world? Will I miss out on a large amount of the game by choosing to play and Aspect?

It is intended that the majority of the game will take place in Eidolon City - especially at the start. The 'real world' angle of the game is there as an additional extra for people who want to play it, and eventually there may be mod-run plots involving events in the real world. However, rest assured that anything that goes on in the real world will have an impact on Eidolon City in one form or another, so Aspects are not going to be missing out!