Photozone shows the resolution for the 18-135 as below the DA 18-250, but that does not correspond with what I found with my copies. Here's the brick wall test. I used a tripod with 2s mirror lockup, shot in AF and cross-checked with MF. AF on both lenses is accurate. Sorry about the whacky exposures, clouds were coming and going.
https://picasaweb.google.com/bonhommed/18135Vs18250?authkey=Gv1sRgCNzC5b_4_qXRqQE#
Click on the magnifying glass, then click on + a couple of times for full resolution. Move the viewing window around in the bottom right corner.
Clearly my 18-135 is sharper across the frame than my 18-250 (and also sharper than my DA L 18-55). The 18-135 beats the 18-250 for flare resistance, barrel distortion and colour/contrast. The 18-250 has better vignetting performance and range. Mechanically, they are worlds apart; the Tamron/Pentax 18-250 is a bit of a rattlebox, with noisy AF and serious zoom creep. The 18-135 has less range but quick-shift, WR, superior build, quick and silent focus and rounded aperture blades.
I no longer use the 18-250 (nor the 18-55) because I prefer the IQ and features of the new superzoom. I paid the same for both, which makes the 18-135 the better bargain. I will cast it aside a minute after the new DA* superzoom (16-85?) is available.