I've been primarily a Nikon shooter for all my digital SLR life; I started on film with some compact cameras and a Minolta X-700, but switched to Nikon starting with a D40, working my way up through the years to a D750 today. I love Nikon glass, I love the ergonomics, and I am very used to SLR photography and all it entails.
But after witnessing the steady rise in mirrorless camera popularity, I decided to start testing the waters with a Sony a6000. And one of the major benefits (at least in my usage) is that if you choose an APS-C system like the Sony a6xxx series, you can (in theory) have a more compact camera system that performs as well as larger SLR brethren.
In practice, you get what you pay for in terms of weight. The physics of light dictate that fast, good lenses will be pretty much the same size on whatever mount you use, and there are always some 'sweet spots' for compactness vs. performance on any kind of camera lens mount.