I can't see any reason (other than price) of why you would go Mk6 over Mk7. It moves the game on in pretty much every area - performance, handling, looks, running costs etc. The Mk7 is holding its money incredibly well for a reason.
Had a friend who had a stage 1 Mk5 and regrets getting his Mk6 as an example. Just replaced most of the suspension on my 180k mile Mk5 and it's like new now, very happy with it. Any reason he's only considering a Mk6 or Mk7? You can find good, low mileage ED30s if he looks.