...that's the big question. Sometimes it can become part of a self-induced stress scenario (that is, "OMG, which camera to bring/use for a given event"). When I reach that point, I try and keep it simple. Knowing that each camera I keep (talk about being a camera hoarder) is capable of superb IQ levels (within its given performance envelope), I will often take two or 3 cameras with me. I then keep in mind the way different cameras render colors (color tonality).
For example, if I'm doing mostly scenics - in particular, in the Fall with all of the autumn colors - I'll reach for my Samsung cameras (i.e. either the NX500, NX300, or NX30). Samsung has a very dynamic color tonality that lends itself well to scenics - but not necessarily for skin tones. If I'm shooting mostly people scenarios, then I'll focus on either Fuji, Olympus or Canon cameras.
Then if I know I'm going to be using external flash scenarios - I'll reach for Fuji or Canon. That's not to say that other cameras don't do external flash well. We just all have preferences with regards to how colors are rendered for scenarios such as scenics, architectural, people/portraits, or general.
The key here (well, it is for me) is that superbly performing cameras of the past usually will have left a lasting impression with their users. Once you get rid of that camera(s), it is only a matter of time before you lament having sold it (them). I've been there in the past and I've regretted having done so - in particular, knowing that the cameras in question were unique (such as the X10 is). So now I'll periodically make it a point of reaching for and shooting with these older cameras and I'm never disappointed, in particular, if you shoot RAW because RAW conversion software capabilities and features are constantly improving.