I had to make the decision OP. I chose to buy now for the reasons most have already stated:
First and foremost: Great truck to drive. Smooth and ample power and transmission, quiet and comfortable, great steering and turning radius, just enough tech for my needs, etc.
The 2017 is the end of very long platform run and bugs are as worked out as they will ever be, parts are abundant, design well mapped, etc. That includes the current EB 3.5 engine, the 6spd trans, etc.
I agree the truck looks a bit dated, but I like what Ford did with the 2015 refresh and think the truck looks classic and will age well from a style point of view. I like boxy trucks like mid 70's K5 Blazers, mid-80's full size Broncos, etc. This Expedition has those elements that scream TRUCK. The new 2018 model, not so much... To each their own.
As others have already said, I will NEVER buy a first model year or anything. I wait three full years for a track record to develop. The biggest problem I have to watch for in the 2017 is bubbling paint on the aluminum hood and rear lift gate and carbon build up from the EB direct injectors.
Finally: price. I was able to get out the door for a great price. If could have waited a bit longer, I would have - but needed to replace a wrecked vehicle. The cost savings later this year should be amazing.