Can anyone tell me what differences there are with a 50mp medium format size sensor compared to a 50mp 35mm size sensor.
This is not exactly what you asked for but it's close enough (50 mp MF vs 36 mp FF):
When you start looking at image quality and resolution the inescapable facts are that the smaller the format the smaller the focal length lenses uses, so the lower the subject magnification and the lower the amount of light passing through the lens. For example take a 100 mm lens at f2.8. It allows five times the volume of light to pass through it at the same aperture as a 24 mm lens. Exposure remains the same because exposure is a function of light intensity ( I think the correct term is really density) and not total volume.
Shoot your landscape picture on an DMF camera at say f8 on a 50 mm lens and you have double the volume of light passing through the lens than you have with an APS-c camera using a 17 mm lens. So not only has your picture been captured on a larger sensor, with more magnification, it has also been recorded with twice the amount of light. The optical resolution of the lens isn't as critical either as you have more magnification.
This is the problem with more pixels on a given sensor size. Pixels are only one part of what you need to realize full potential resolution. I guess this is why Canon haven't been in a great hurry to bring out a 'very high mp' FF sensor. Incidentally I believe this is the reason we don't see the full 'reach benefit' of crop sensors. When you are reach limited and use a crop sensor as opposed to cropping in on a FF sensor ( resulting in less 'pixels on target'), the only thing you are benefiting from is more pixels. The magnification, volume of light, lens optics etc. all remain the same. So you don't realize anything like the amount you think you should.
Now's probably not the right time to say I'm thinking of getting another 5D mark 1.