BRU is a delight to leave from, comparing with FRA or LHR. A variety of lounges, straightforward security, sometimes a longer walk but no big deal. It's also good for arrival, though there is no arrivals facility.

FRA is much easier for that, as the AC can be used for arriavls with shower, etc., the hauptbanhof offers incercity trains, and S-bahn station for local rail - a true intermodal transportation node! But as Stefan says, brutal for departure. The Frankfurt zoo is amuch more reasonable (and recommended) experience.