1) I've found the mirrorless system works better for my current shooting style/time. I travel for business a lot and the M3 and the lenses are easy to carry either in carry-on or when driving.
2) The EOS-M3 can use my remaining EF lenses with the adapter. I did sell my 70-200 f/2.8L as it was overkill with the smaller camera.
3) The EOS-M3 can use the new electronic viewfinder (which I purchased) which eliminates the shooting with my arms in front of me (which I hated)
4) When I went on vacation recently, I decided to leave the 5D3 at home because of the weight and the fact that I was shooting with the G1x and original M 99% of the time.

It is taking some time for me to get used to the new system but I had a fun morning taking some train shots today and I'm really enjoying the compactness and capability of the new system.

Here's a shot from this morning... edited in my normal style.