the biggest benefit of alfa wheels is the direct fitment. there is quite a good range to choose from, whether they be teledials, multispokes, Blackline, etc.
whilst aftermarket wheels opens up what seems to be a huge choice in style, finding a set that fits is a big difficulty, and may require compromise. in particular, alfa's 5x98mm PCD bolt pattern is unusual, and you may need to go with 5x100mm and use 'wobbly bolts'.
there is a fair amount of info in the forums already about the other specs (offset, centre bore, tyres, etc), so research carefully and ensure what you buy will fit, be the same rolling circumference, and not rub.
if you go with 18s or bigger, lowering the car is almost a 'must' for appearances, to avoid the illusion of the air gap between tyre and wheel arch appearing too big.