Scott, I deliberated over the same question when I put together a Pentax 67 kit (nearly 20 years ago!). As primarily a scenic
photographer, I had usually found the 50mm lens in 35mm format to be somewhat tight, so I favoured the wider or 'looser' view offered by
the 90mm lens. And the 90 fitted better with my 45mm, 165mm and 300mm. In addition, I reasoned that the 90mm lens was more
compact in size, focused quite a bit closer, and, as the much newer design of the two, might be sharper. I bought mine new, and was
certainly pleased with the results it gave me. However, as I came over time to do more portraits, at times I wished for the longer
perspective of the 105mm. And the 105mm is no slouch in sharpness either; I recall the 1999 (I think) test by Popular Photography rated
it as 'excellent' at most apertures and a pro told me that he considered his 105 to be very sharp indeed. Certainly, either lens will have better contrast than your 135 macro, plus they're much more compact and provide brighter viewing. Make sure you get a later production copy with the Pentax 67 designation and check it against a bright specular light source for hazing on the elements which seems to afflict many lenses, I think due to either humidity deposits or heat volatilising the helicoid grease. I sold most of my kit when I eventually switched to the Mamiya 7 system but bought a like-new 105 as a portrait lens for the body I kept, for a low price. It's a very nice lens and does offer slightly brighter viewing and the shallower depth of field.