If you're talking about fender gaps, then it makes no difference. Both have the same diameter tires so the "wheel opening" is filled the same amount in both cases. Now... if you're referring to the amount of that area that's filled with wheel vs. with tire, then you have a point. That's why people tend to like the look of bigger wheels. Wheel is prettier than tire.I think the 19s fill the wheel opening a bit more and therefore like them and do not feel much in compromised ride with them.
I'm 68 and I prefer 20" wheels.Yes, and to reinforce the age thought, I am 68 and therefore prefer 16" wheels . I had 18" on my 08CS, 19" on my Cayman R and 19" on my 981. I think the 19s fill the wheel opening a bit more and therefore like them and do not feel much in compromised ride with them.
have a 2015 Catman GTS, cane with 20" wheels, very few DE tire options...so I put my pals 18" 2009 Cayman S wheels on today, his fronts are 8 1/2 " wide, the rears are 10".O.Z. Allegrittas and running Hoosiers. The front tires have about 1/2" of clearance to the front struts, the rears look so tiny.....I am going to 19", 235/40/19, 265/40/19...maybe even a 35 profile.Personally I like bigger wheels more. Could be psychological, but bigger wheels feel more responsive. Air compresses, while metal doesn't. But I really wouldn't wanna run anything below 35 profile, or maybe even 30 at the absolute most on these cars.
Some argue smaller wheels have less unsprung mass, but that's something hard to measure really. You could argue that the smaller wheel has more mass closer to its center, while bigger wheels have their mass stretched closer to the end of the tire. Weight-wise, the 20" might not save you much as you do end up pumping more air.
I'm sure someone has done it, but it's worth a look to weigh two wheels of same design (one 1" smaller in diameter) including having their respective tires pumped up to same pressure. I know weight doesn't mean a whole lot compared to unsprung mass.
Some do go into smaller wheels (i.e. 18") just because of cost and tire selection due to track use.
Something else to consider is rotational inertia. Extending the mass an extra two inches from the center of rotation must make it harder to accelerate and turn that spinning assembly, though I have no idea whether that effect is greater or less than the reduction in unsprung weight.So i actually weighed my 18 inch rims (with tire) and my 20 inch (with tire). The 18s were the cheapest rims you couod buy a 981, the 20s are the carrera classics style 10 spoke alloy rims.
The 20s weighed LESS than the 18s. It was about 3lb in front and 5 in back if i remember right. The 20s were lighter.