Any real difference? In your experience which is a better performer?

I'd like to plant them in 3 large window box style planters that sit on my front porch. Faces east, gets morning sun.

They are pretty large planters and I'm thinking it will be rather expensive to fill them all with just the superbells. Can anyone think of another white flower (I'd like to try a moongarden type look) that is on the less pricey side and would mix well with the trailing millionbells? I was thinking annual vinca.... Or are they too similar in shape? When filling a window box should I go with a larger type flower to contrast with the millionbells? Thanks in advance for any advice!