The 90mm f/2.8 is the sharpest Pentax medium format lens I own (the others are the FA early 35mm, the 45 2.8, 50 3.5, 75 2.8 (also a great lens), 45-85, 120 macro (excellent), 80-160 4.5, 150 2.8, 200 4.0, and 300 4.0) I've used the 28-45 and it's wonderfully sharp, although heavy. The only disadvantage of the 90 is that it's heavy and expensive, but it's also weather-sealed. Don't overlook the 75 2.8. It's quite sharp and both small and light.
David V. Ward, Ph. D.