I own and use one of each, and still use both of them. I do realize better throughput with the newer unit, but not appreciably so. My advice is that the older unit is still very serviceable - I see pretty much no advantage to buying the newer unit if the older model still serves your needs.
I only bought a 6th gen AE because I needed another AE and the older unit had been discontinued at that time. And, I found the 6th gen AE at a steep discount at Staples. Both my AEs are solid performers, no regrets.
My next wireless router is likely going to be Synology RT2600ac, with the sale of my two AEs pretty much funding its purchase. The RT2600ac has so much more going for it, the only knock being that it's pretty fugly…