I use 3 lenses on my 12STC with IQ180: Rodenstock HR 35, 60 and 100mm lenses. If I only had one, I'd choose the 60mm and stitch for wider coverage.
Remember that the perspective of your chosen lens is slightly affected by the sensor dimensions, and you did not mention which sensor you'd use. With a smaller sensor dimension, I might go with the 35mm.