Due to the pollished/lac finish to the monza shadows they will suffer in long term use to the dreaded white worm/corrosion issues were the lac lifts and leaves white paches. The painted finish to the 17s are a much longer term solution.
But from past experience on car forums alot of owners are after looks so go for the shadows/monza IIs or simply don't keep there car long enought to worry about it or don't use it as it should be and spend most there time cleaning it and keeping it locked up in a garage lol.
I went in the end for the 17s for a couple of reaaons. 1) I have a set of monza IIs from our mk5 that i can use if i want (put it back on the 17s to save the wheels from out lass!) and i much prefer there classy look of the monza II to the more halfords aftermarket barryboys looking shadows. *all my personal opinion. 2) I also fancy a set of scirocco wheels alot its just the different offset and 0.6" sticking firther out/maybe handling issues that brings it keeping me from them.
Thank God for car warranty ehh...
3 years time + white worm = brand spanking rims
. Considering I change my car every 3 -4 year this wont be an issue for me.
My vote goes for the 18's, no comparison over the 17's. I do like FroGTI's rims though