Borders - How Do You Decide?

One of the things I struggle with is deciding on borders for a quilt.  Most of my quilts are pretty busy, so I tend to go with plain (though usually print) borders.  I struggle with the width of the borders.  I have been using 1" or 1.5" inner borders in a dark color (generally).  The outer border, though, makes my head hurt so I usually just eyeball something or use what the pattern calls for if I am making it from a pattern.

So how do you decide?  I've seen potential formulae based on the block size in the quilt - never less than 1/2 the block size and no bigger than the block.  But then is it the largest block in the quilt, or the smallest?  I make stuff that doesn't necessarily have well defined blocks.

Here's an example:

I used a 1.5" red border to stabilize and ground the quilt.  Now how should I decide on the outer white border?  The smallest unit is 3" and the largest is 5 1/2". (And I must say this is after consultations with several folks!)

Narrower .... or .....

Wider?

I thought I would like the wider border which is 5", but seeing the picture makes me think the approximately 3.5" white border might work better.

How do you decide?