I have the 16-45, and I got it because of the 2mm advantage over the 18-55, more than for any image quality difference.
But now I have the 10-20mm (sigma) so it turned out not to matter much. But if I hadn't gotten something wider, 1mm would matter. I'm assuming it's actually 1mm - I haven't seen both lenses (the pentax and the sigma you're considering) actually measured, so you might look into that.
I have the 16 -45 and 55 - 300 and it covers things quite well. Just to throw a spanner in the works, have you considered a prime - the 35mm f2.8 macro is a beauty.