If you don't need a bigger car, you're better off looking for a good Ed. 30 or a mk6 R.
A good Ed. 30 would only compensate if the price difference isn't too big from the price you can sell your GTI.
The mk6 R has AWD and the engine has a better base to work with, so it compensates the higher price if you're looking for an AWD car.
The Ed. 35 is too expensive IMO to enter the equation. It only has a new body and interior, the rest is the same. You can upgrade the interior to have most of the toys you can have in a Ed. 35. I prefer the design of the mk5 so the mk6 GTI wouldn't be an upgrade.
You can always retrofit the parts that your GTI lacks, like a better engine or upgrade your own engine.
