MTS

I think the way the fabric blends is more important - so the solids chosen should be from the florals, with not a lot of contast.
You don't want a lot of 'background' - busy is better.

But you need to watch the scale as well.
Depending on the patch size you use, it's possible that a huge Kaffe fabric won't work at all - despite the busy pattern - because when you cut it up for the HST, you'd end up with a solid patch - you want the patterned fabric to carry out into those HST's as much as possible.

Of course, the converse is true as well - you don't want a very small calico type print either - again, even though it might have multiple colors.

I think you would pick the florals first - and they should have some commonality in style, tone and color.
Then pick the solids. And I say solids but I really mean blenders.
This is not the quilt for harsh Kona solids. You want some texture or tone-on-tone pattern in the complimentary fabrics.