I have owned a Yashica 124G, a Mamiya C330f, A Yashica Mat and a Rolleiflex 3.5f; the latter two I still have. I found that the lens quality of the two Yashicas was very good and there was little difference between the two ( they use the same lens but the 124G has a built in light meter and is much more expensive). The Rolleiflex is a wonderful piece of engineering and gives superb results. The Mamiya has the advantage of interchangeable lenses and macro but is very heavy and not really suitable for carrying around being more at home in the studio.
So my opinion for what it is worth is:
The Yashica is a delight to use, gives good results and is fairly cheap to buy.
The Rolleiflex is exceptionally well made and gives great results but a good one is expensive (for a 40 or 50 year old camera) but will probably increase in value if well looked after.
The Mamiya gives good results and is the most versatile but is rather large and heavy' however they can be picked up for a reasonable price.

You can see photos taken by the Yashica Mat and the Rolleiflex on our web site at :

http://www.jennyandernie.co.uk/Old%20Cameras/TLR.htm

Ernie