For serious travel photography, would you rather use an xe-1/xpro1 or a canon or nikon full frame?

Short walking trip (a few hours and then back to the hotel) or driving trips - I would bring both.

If you are serious enough to take the images as part of your portfolio, I would bring the DSLR as my main gear and then a mirrorless as a back up or side arm. However, if your DSLR is an entry level type I suspect the difference between the two is small. It all depends on what lenses to use.

If it is street / snap type photography a mirrorless would be more handy. If you don't know the local culture and the crime rate, the mirrorless is also a better choice.