trying to decide on wheel sizes

Caravanshaka

That's the key, aspect ratio:

think of it as the sidewall height = (aspect ratio) x (width)

both wheels are 20 inch, so you want the same sidewall height on each to achieve the same overall diameter

for a 255/30, height = .30 x 255 = 76.5
for a 305/25, height = .25 x 305 = 76.5

hope that helps