my guess is GR7 should have newer and fancier effects. It's always been a great effects tool.
As someone who uses Mixbox a lot, I would say, it makes sense to get it if you are a user of TRacks. It's very handy to have access to those modules all in one slot. In my case, I am a big user I got pretty geeky with it. I would read the IKM manual and then I dig up other articles on the units it emulates and go in there and tinker to see what it all can do. . So I am familiar with the stuff I like to use. I have other stuff in my collection, but defer to Mixbox as its much lighter on CPU compared to some other stuff i have. YMMV.