They're both such bold plants, foliage and flowerwise, I would go with something really airy. And let's add, fragrant foliage while we're at it... Calamintha nepeta. Great plant which seems to be undemanding and looks great with most anything. However, the picture doesn't the color right, its more of a purple. Last, I asked last year if this was invasive because its a member of the mint family, I got a resounding "no". Just thought you might wonder.

Hope this helps.

Here is a link that might be useful: Calamintha nepeta