I dont know which merc your looking at, but basing it on the C180 Komp Classic, Glasses says the following....
P/X £8600 excellent, £7750 good, £6810 fair. Dealer retail £11200
Now if your Golf is a 2006 06 GT TDI, the figures are
p/x £9190 exc, £8280 good, £7370 fair. Dealer retail £11000
Therefore, although Im assuming a lot about the cars as I cant confirm the model, the Golf is worth £500 across the board more than the Merc come trade in time.
Hope this helps
Andy