I know this is a little late, but perhaps it will help to reaffirm your decision. Personally, I find that the diagonal overflows look much better than the corner box ones. Of course, this assumes that the diagonal overflow doesn't jut too far forward. I think the digonal overflow is best used on fairly wide tanks (24" preferably) but would work out ok on an 18" tank as long as it was kept as small as possible. But I'll say it again: I think the diagonal ones look better than corner box ones because they're more continuous.
So, good call on the overflow, and show us some pics when she's done.
