If you are using both lenses with the same digital sensor/back, the image produced by each would cover the same field of view, as they are both 28mm lenses (there are some caveats, including numerical rounding, floating elements, distortion, etc).
Rodenstock provides the diagonal viewing angle based on the circular image produced by the lens (focused at infinity). They use the image cast by the lens, in part, because they expect their lens to be used with many different sensor sizes/shapes and also, importantly, shifted. Few, if any sensors, are large enough to capture a 101º viewing angle with the Rodenstock 28mm in one capture; However, it could be done by stitching multiple shifted images. If you used the lens unshifted, the sensor would be sitting within a large circle, and there would be a projection of an image all around the sensor that wasn't being recorded. Essentially, you would be cropping the image down to the sensor size.
The HCD lens is designed to be used with an H-series back, so Hasselblad provides the viewing angles based on the H-series sensor size. On the datasheet, they say it is designed for sensor sizes of 37 mm x 49mm (
link). So, the HCD describes it's viewing angle based on an ~61.4mm image circle, while the Rodenstock is based on a 70mm image circle (
link).
The HCD lens probably casts a slightly larger circle than the sensor, but it is designed around that ~61.4mm. How do you define how big of an image circle is cast by a lens? Arbitrarily... It's up to the manufacturer to decide based on their own criteria how large of a sensor/circular_image the lens is designed for.
I suppose one takeway is: if you want a wide field of view (and you aren't planning on shifting the back), shorter focal length determines which lens will produce a wider field of view.
I hope that helps...
Details
Actual focal length: Hasselblad is good about stating the actual focal length in their datasheets, the HCD28 is actually 28.9mm (
datasheet). I didn't see it in Rodenstock's data sheet (
link), but I think the Rodenstock is very similar at 28.8mm (based on a quick estimation using their 70mm image circle and 101 degree viewing angle).
The calculation is (note there is plenty of rounding in data sheets that can matter here):
Diagonal_Field_of_View = 2 * ARCTANGENT (.5 * Diagonal_Image_Size(or Diagonal_Sensor_Size) / Focal_length )
Don't forget to keep the units the same (generally mm), and to compute the arctangent (inverse tangent) using degrees.
But it may also be worth looking at Rodenstock's 35mm. I never hear much about it, but it's MTF is among Rodenstock's best. It's a narrower field of view by about 10% in any one axis, but it's a bit lighter and cheaper. Hasselblad also runs a HC35, which should cover the H5D60 too (