As far as power, the 420 and 360 are comparable with the edge to the 420. The 420 has a longer reaching zoom head that helps it get light farther, hense the higher advertised guide number. The swivel head is a big plus imo, but that's subjective.
Doesn't mean Canon trumps your Pentax tho.
Pentax has better Wireless flash capabilities tho (the 420 can only be used as a slave flash unit). The Pentax also has better Manual Flash capabilities (the 420 requires the user to input a shutter speed and aperture, then the flash calculates, where as the Pentax can find the desired fraction of power - a true manual flash).
Probably more differences, but that's enough out of me.
And the 360 is cheaper than the 420