My opinion is the following.
Black wheels & black car = lets say that gives a more underground look, low profile.
black wheels & white car = the difference between the 2 colors is well known and you wll "make a difference" for the eye to see.
As for selling a car. if you have 2 ibizas' and you have in mind of keeping as a main car one of them, I would advice you to keep the second one for spare parts. usually the price of a car itself is less than selling it in parts. Also you might need something , sometime and you will search to find it like crazy.
Lets face it, our car is getting old and parts are hard to find.
even Seat has to search sometimes.
I asume that you have 2 MKII is that right???