I have a Dx7 mk1 that I just picked up off craigslist less than a month ago. The big difference is that a dx is just FM and it has 6 operators.
check out the an1x manual on page 59, the algorithm section shows you how the fm works.
http://www.deepsonic.ch/deep/manuals/yamaha_an1x_manual.pdfthe an1x is a VA with fm capabilities and you can get very fm like sound of course, and you have a lot of options on whats modulating the fm stuff and you have a filter ect.
the dx7 has no filter and you are doing nothing but fm, the dx7 to me sounds way more sega genesis sounding than the smooth an1x va sound. The dx also has way more room for crazy experiments, since its not like subtractive synths. so it offers a new different way of doing things if you are only used to subtractive. also a computer editor is a must, its hard to keep track of everything thats going on in a patch on the crippled dx7 interface. I kinda almost wish I had a tx7 because of how much room the dx takes up, but i got one that has an E! mod so it has 10x the patch storage.
Its really up to you, and if the dx/tx is the sound you want.
-sean