Roger Said the Following: >>> " 4. Rendering....each lens has a character . This is one of the joys of shooting with a M system. You can range from bold color,high contrast,razor sharpness to softer color,natural contrast and creamy bokeh.
So if I have to pick just one for the long term........its the 90 summicron"<<<
I am assuming Roger was referring to the 90mm AA?
In either case Roger and most everyone expressed valid and excellent reasons on which possible lens to select. For myself it was a difficult choice as rendering is what makes the Leica M system really stand out as well as sharpness. Considering just the 75mm and 90mm focal lengths (and just leica lenses for the moment and putting aside frameline accuracy)...we have the the 75mm Summarait and Summicrons for when ultimate sharpness used wide open is a prime consideration and the 75mm Summilux f1.4 for when somewhat softer rendering (contrast, sharpness etc.) is desired, especially for portraits and certain trypes of atmospheric types of images. The same sort of logic can be applied for the Elmarit-M 90mm f2.8 (last one produced), 90mm f2.5 Summarait and 90mm f2.0 cron AA....all close to being equally sharp wide open vs the 90mm Cron f2.0 pre AA (especially the last one produced) which is equal in size and handling as the 90 AA) but which has a somewhat softer rendering at f2 and f2.8 than the other three lenses but keeps up with them in sharpness when stopped down.
In the 90mm catagory, especially when speed is a concern, you'll find just as many suggesting the 90mm AA as you will the 90mm pre ASPH. I could go back and forth all day which one, as it all depends on the specific use and character I'm looking for. Although I have the 75mm Lux for speed and softer rendering and initially went for the 90mm AA as a balance to it (spfter rendering vs. ultimate sharpness)...I kept gravitation to the 90mm Pre ASPH (sold the 90mm AA)....and yet, there are times I would have wanted to reach for the 90mm ASPH for its sharpness wide open...but not always. It told me (for myself specifically)...that the slightly softer rendering came into play more often than not, although I shoot subjects in lower light, where great sharpness and rendering of detail is important. YOu can make one work in pl;ace of the other..but not always.
I think first you should determine which focal length works best for you. As mentioned, if you have a M8 (with it's orginal framelines) and a 50mm, then the 90mm would be my next choice. From that point I would determine if you need the extra stop of speed of an f2 vs. a f2.8. If f2.0, then rendering is your next important criteria and will determine which 90mm f2.0 lens...the AA or Pre ASPH lens. If f2.5 and f2.8, slightly slower and smaller handling lenses are fine..and great sharpness wide open works for most all types of situations, then these two should be considered and like everyone has said...the f2.8 is a top choice for its all around performance, value, handling etc.
Lastly Mike also had a interesting suggestion that if you were to contemplate a 135mm at some point, then the 75mm might be your next choice as 90mm and 135mm might be too close and it provides better spacing. This is absolutely true, yet for myself, I view the 135mm in a catagoy all its own if using a M8 or M8.2, as it brings a efective field of view of approx 180mm...which will generally only be used for quite specific applications. The 90mm will have an effective field of view of 120mm, which still keeps it in longer portrait territory or "longer street work" type of photography..and so, for myself, I would for the moment, keep the 135mm and its future purchase and use out of the equation and decide on the 75mm and 90mm focal length question ny itself......but thats just my personal way fo things based on my own experieces.
So much great and helpful advice here and as costly as it may become at times...trying out and seeing what works best for you both in focal length, cost and most important the rendering of these optics...will ultimtely help you end up finding exactly what right for your style of photography (as well as budget).
Dave (D&A)