I researched the Edge and MKX for a while and although they are not a whole lot different from each other, I like the little luxury items included with the MKX and the 2 years and 14,000 miles longer on the bumper to bumper are nice also.
I traded in a 2006 Navigator for the MKX and I had a 2004 Cadillac SRX before that and so far the MKX is the most fun to drive with the SRX a close 2nd. I would have kept the SRX but I had nothing but problems with the huge sunroof. The MKX (now) and the SRX (then in 2004 first model year) seem to be drawing the same curiosity when people see them.
I took a bath on the Navigator trade but at about 7 - 9 MPG better, it was worth it. Not to mention the $200 less per month payment. I put 20K a year on a car so between, gas ($150 less a month at $3.80 a gallon), payment ($200 less a month) and my lower insurance cost ($300 less a year) I'm saving almost $400 a month.