I have been a little under the weather - thus my late response. Here are some subjective impressions. Both have strengths and weaknesses.
Wide-angle. In this category Pentax shines with the new 25mm and the 35mm A and FA. These are very well corrected lenses. In contrast, the 40mm/3.5 and 50mm/2.8 Superangulon lenses are also very sharp and well corrected, but suffer from pincushion distortion, which may be easily corrected, but is less appealing at first sight. The mustache distortion of the 40mm Zeiss is even less appealing. Pentax' 45mm is not well suited for digital. If I know I will need a good wide-angle set-up, I take my 24mm XL mounted on a Horseman and a Sinarback in addition to my Hy6 set-up (heavy).
Standard focal length. In this range, both Pentax and Rollei have excellent options. The Pentax 75mm and Schneider Xenotar are both superb, with the Xenotar perhaps a tad sharper. Pentax' new 55mm is excellent as well (perhaps the first batch was of a lesser quality). In a similar range, Rollei has the 60mm/3.5 Curtagon and the 55mm Shift Superangulon, all outstanding.
Macro. Both Pentax and Rollei have great options, and I don't have the new Pentax 90mm, which is apparently receiving lots of praise. Pentax' 120mm A and FA are fantastic lenses. Nevertheless, I'd say that Rollei wins in this category hands down. The 90mm/4 Apo-Symmar is incredibly sharp on digital backs. The 120mm Zeiss, while just acceptable at infinity, is a very practical and very sharp macro lens. There is also the 150mm/4.6 Apo-Symmar which allows for longer shooting distances. Finally, the 60mm/3.5 Curtagon can be mounted in reverse mount with automatic diaphragm etc.., extending the Hy6 into the micro range. Thus Rollei has a wide range of four just superb macro lenses, while Pentax has two.
Portrait. I'd give this one to Rollei as well, at least when comparing systems lenses. Both the 110mm/2 Planar and the 180mm/2.8 Tele-Xenar are portrait lenses without parallel. The Schneider 150mm Tele-Xenar also generates a beautiful image. The 150mm/2.8 Pentax is also a great lens, but less exciting than the Zeiss/Schneider offerings of the Rollei.
Telephoto. Rollei's 500mm options, either the Tele-Tessar or the Apo-Tele Tessar are offered at f8, too slow for practical purposes with medium format back. Here the 300mm/4 Apo-Tele-Xenar with the 1.4x Longar provides a superb combination with stellar sharpness. Alas - Pentax wins hands down with a broad line-up of four telephoto lenses, all of them well corrected for digital backs, the 300mm/4 FA, the 400mm/5.6 FA, the 600mm/5.6 A, and the 800mm/6.7 for the 67. There is also the 150-300mm/5.6 zoom and the slower 300mm/5.6 (which I don't have). Finally, I use the P645D with the 1000mm/5.6 Carl Zeiss Jena on a regular basis.
Specialty lenses. Many lenses can be easily adapted to the Pentax mount, including vintage softfocus lenses, macro lenses, etc.. This adaptability substantially expands the range of the Pentax system.
Overall image quality. From a subjective perspective, images from my Hy6 with 75 LV Sinarbacks appear sharper and more brilliant than images from my P645D with Pentax lenses. Don't really know the reason, but the leaf shutter might play a role in this. However, I'll usually have more keepers with the Pentax as the camera is so much more user friendly than the Hy6. Once you nail the shot with the Hy6 and some of those brilliant Zeiss and Schneider lenses, there are few parallels.