The question is simply – do you need the extra processing power or not?
You buy what you need for the job, simple.
If you’re just editing standard HD footage (1080p max) and only doing relatively low intensive processing effects then a top of the range iMac is absolutely fine which is what I worked on for a while.
However, as I progressed as an editor and motiongraphics designer I found myself working on 3D jobs and working with 4k footage (4 times the size of HD and more than 4 times the file size).
It quickly became clear the iMac couldn’t cope with the jobs I was working on and even now my MacPro needs upgrading to cope.
If you don’t anticipate working on anything over standard HD or doing any 3D rendering then just get the latest iMac but if you think you might work with 4K footage or do some 3D rendering then I’d hold out for the new batch of MacPros to come out with the new intel processors and hopefully thunderbolt compatibility.