So my question is how do you decide which to take with you for anything from a local walk to a trip around the world. I have seen myself pack and repack the camera bag half a dozen times before leaving for a trip. No matter what I take I find I am usually willing to live with my choice. On a walking trip across Northern England, I carried one camera with a fixed 28 FOV lens. A year later I took a digital Leica with 35/90 combo and was happy with that combination. I tend to carry digital for travel just because the results can be accessed so easily.
The answers are purpose and situation.
If it's for work, I choose the most appropriate gear for the job. This means the Panasonic G9 for video and portraits, 5D Mark II for marketing images, Canon 30D with Sigma 18-35 for a bit of action photography that will go online.
If it's for fun, I think about where I'll be shooting. Very casual shooting only requires a pocket camera, which is now the Sony RX0. More dedicated shooting or the potential or portraiture with family and friends means the M9 + 50 Cron, or m43 camera with 25/1.8 and 45/1.8. The greater need for discretion, the smaller the camera I choose.
Travel is a somewhat different story. Overseas travel is usually one of those once in a lifetime journeys, so I take gear with the best ratio of size, weight, quality and versatility. This means the M9 + 21, 35 and 50, and a m43 camera like the G9 with the Panasonic 12-35, Olympus 12 and 25.
If I'm pairing cameras during travel, one camera gets a wide and the other gets a normal lens, so M9 + 21 and G9 + 25mm (50mm equivalent) for example. Another possibility is the M9 + 50 Cron with the Sony RX0 mounted on the hot shoe. This gives the ability to get a decent quality wide angle shot very quickly without switching cameras.
Before any trip, I spend many enjoyable hours contemplating what gear I will bring, but often fall back to the well tested stalwarts when the time comes.