Just noticed this and thought I would give you a few things to think about.
What are the main things that you intend shooting (Landscapes, portraits, Action)?
Having used DSLR's (Canon 40D, 60D, 7D, 5D MKII and 5D MKII) and moving over to mirrorless about 2 years ago (Olympus E-M5 and now a Fuji XT1) I have experienced pro's and con's with both.
If you intend to only mainly shoot static to slow moving subjects then mirrorless may well be the way forward but no so much full frame.
Even high-end mirrorless cameras (over £1000 for a body only) will still no compete with even a mid-range DSLR (Under £500) when it comes to tracking relatively fast moving subjects. If you are only shooting portraits,landscapes etc... then you should have no issues with either system.
The issue regarding the requirement for a full-frame camera is one that needs to addressed separately. Why is it you have the requirement for a full frame sensor?
Are you shooting a lot at high ISO or wanting extremely shallow DOF? Do you intend printing large (over 16x12)?
What budget have you set for glass?
Weight is also a factor here. Yes a mirrorless body is smaller and lighter than a DSLR however fullframe glass is heavy which ever way you look at it.
The difference between the body is only going to be about 500g the rest is going to be made up of lenses and extras.
If you do not need some of the above then I would really consider looking at some of the APS-C mirrorless cameras that are on the market now. They are lighter and more to the point so are the lenses. My 5D MKIII and a selection of glass used to come in at just under 9KG, the same focal range with my Fuji mirrorless comes in at under 6KG (a BIG difference when you are lugging it around all day).
Just a few things to think about, I have also attached 3 images. One was shot on a full frame 5D MKIII, one on a m4/3 Olympus and one on an APSC fuji XT1. Can you tell which once is taken with the 5D MKIII?