The diameter of the wheel is not "wheel base".
Wheel base is the distance between the front hub and the rear hub centers.
To answer your question however; The total diameter of the wheel and tire combo is what matters.
Normally the larger diameter will offer better turn response and feel.
There are a few things which affect "handling" that you need to consider.
1. overall weight... lighter is better, but you're still changing the spring and strut design application.
2. Ride height change... CG and "centers"
3. Brakes and braking.
possibly a few I missed.
There is no benefit to going to a 19 inch rim.
It's been tested on other cars a few times by magazines and if i remember correctly, once a car reached 18's, there was almost no difference in handling to a 19.
Lets not forget too, 19 is an oddball size, most good tires are not made in the 19 inch diameter.
Last, but not least, 18's on a fox leave you pretty low on sidewall, 19's would be worse.