18s are diamond cut, 17s are normal powder coated ones...on FR, on the Xcellence, the 17s are diamond cut version of FR's 17s. Both are 215 sectioned tyres, with 17s at 45 and 18s 40 profile. Physics would say that the 17s would be marginally more comfortable, quieter and fuel efficient than the 18s.
Where as the 18s look really nice, it also exposes more the tiny brake disc sizes on the
Ibiza, which to me is a bit of a mismatch. On a more performance orientated car large wheels are required to house the large disc and calibers under.
Oh and diamond cut ones are more of a pain to maintain and if you curb them frequently, expect an expensive refurbish. Else suggest using alloygators.
I guess it would be difficult for others to suggest road noise and ride quality unless they have driven both on a test drive, or own both (which is unlikely). Myself, I had a
Mk5 Ibiza with 215/40 R17s and test drove a Mk6 on 215/40 R18s, and I felt the noise and comfort were decent considering the profile and size of Mk6 tyres, def better than the MK5.