Originally Posted by yaw

I have 19.5 internal rims and opted for 25 over 28 GP5k on the front for the aero profile because the 28 bulged out and the difference in comfort is not noticeable enough for me. But I use 28 on the rear.

25 front measures 27
28 rear measures 29
(after. ca 1000km)

So if you're mostly on good surfaces, I'd go 25 to give the wheel a better frontal profile. But if you otherwise need the extra width or lower pressure then 28.