Well it also means the ROI is slower, but you are right, it depends on your personal constraints (electricity rate/space available/heating dissipation). Also keep in mind that nvidia cards can only mine equihash at this efficiency, right now.
Also phil did you manage to mine with AMD and Nvidia cards on the same mobo ? It should work but you never know

yeah but You need a better cpu not a cheap celeron.
In my case summer mining with amd is a no go.
I have never seen a five card amd giving 1500 h at 600 watts at the wall
but I have seen a four card amd giving 1210 h at 570 watts at the wall.
I can do 1800 h at 570 watts at the wall with the 1080 ti (3 pieces)
So I added about 600 h for no watt cost.
and here is the lowest price yet for a 1080 ti
http://galaxstore.net/GALAX-GeForce-GTX-1080-TI-Founders-Edition_p_123.html659 + 15 to ship to NJ = 675