You can find an excellent set of reviews by Jack Flesher here:http://www.getdpi.com/forum/lens-accessory-reviews/15710-mamiya-lens-overview.html
Here is a quick summary of the major generations of lenses along with their common abbreviations:
- Manual Focus Mamiya lenses ("N", "MF" or "Pro TL generation")
- Mamiya Autofocus Lenses ("AF" or "non D")
- Mamiya Digital Lenses ("D")
- Phase One Digital Lenses ("D")
- Schneider LS Lenses ("LS")
Note you can also use Hassy 200 and 500 series lenses and Pentax 67 and a few other lens varieties via adapter. You must control the aperture stop down manually in such cases, but gain autofocus confirmation.
I'd strongly (and selfishly) suggest you work with a knowledgable dealer that you trust to give you experienced and honest advice on these lenses. Personally I try to do as many of my own lens tests as possible and give straight forward feedback from them. See for instance our comparison of the Schneider 28LS and Phase One 28LS: http://www.digitaltransitions.com/blog/dt-blog/28mm-32mm-test
where we show the 28D and 28LS perform very very similarly.
In many cases the difference between generations is startling. In other cases the difference is nearly non-existent. The latest-and-greatest Phase One 120mm Macro AF lens is not that much better than the was-already-excellent Mamiya 120mm MF non D. So if you don't need autofocus you can save a lot of money by going with the older lens in that case. There will be a bit more chromatic aberration and of course there won't be a manufacturer warranty but with this kind of lens there are very few things that can go wrong with it. Similarly if you don't need fast flash sync (which some people really need, and others will have no use for depending on what they shoot and their style of shooting) I'd suggest the 150D over the 150LS
as the 150D is an amazingly good lens and is f/2.8 rather than f/3.2 (though it is a bit heavier).
On the opposite side you have lenses like the Schneider 110LS and 240LS which are just stunning and big improvements over the closest equivalents in the previous generations.