Well looking at the data for the Pentax lens, vignetting (very little and less than the Canon), distortion (virtually none) and chromatic aberration (minimal) are nothing to worry about.

The main thing to compare is the resolution. They are measuring this in Line Widths per Picture Height (LW/PH). Think of this as measuring the limit the lens can resolve (distinguish) very narrow lines close together before it just becomes a grey blur.

The Pentax lens maxes at around 2400 (LW/PH) in the center, the Canon at 3300. The figure of 2400 seems to be an very good result, as I see sharp macro lenses (Nikon 105mm for example) in that same range. The Pentax is almost as sharp in the corners, whereas the Canon is softer in the corners than in the center.

So the Canon lens appears to be able to resolve more detail (more distinct lines, 3300 vs 2400). However Photozone also say the "visually relevant" range is up to 2250 LW/PH . By that I take it that beyone 2250 the lens will resolve more than the sensor can record, or more than the human eye can detect in normal use. In fact the Nikon 300mm f/4 has a LW/PH value that maxes out around 2100, and it is called "very good to excellent"

I'm not an expert on MTF charts but it looks like optically the Pentax lens is very good. I guess you'd want to also evaluate the build quality, weather resistance, auto focus speed.