17's are the better option on cars with the basic rear suspension (150PS or less). 18s on those with the independent rear suspension.
Having driven both options alongside a former Aston Martin development driver it was clear that the smaller wheels and hence slightly higher profile tyres have more compliance. That really helps control rear end float when you're pushing on and the road surface is anything other than track smooth. 18s were very lively on the rear, and clearly compromised the handling, ride difference wasn't huge between the two, but noticeable.