I use both the P67 system and the Mamiya 7 system. The Mamiya is indeed more compact and a lighter burden but the actual difference is very little compared to the support system differences required for each. The Mamiya is easily controlled with a good, light-weight tripod where the Pentax requires quite serious and bulky tripods to work properly, even with wider lenses. The Mamiya wide angles are wonderfully distortion-free and will bring back superior images in more situations. The Pentax glass is very good but the Mamiya rf optics (especially the wides) are in a different league, in my experience.
Something I also found is that because of the ease of carry, I actually have the Mamiya with me. The P67's and heavy pod got easier to leave behind as the years passed and I missed some good opportunities just for that simple reason. I wouldn't fret the filter issue as a little practice will get you tuned into that.