In your situation I would prefer the 90mm Summicron. The 75mm Summilux is a bulky lens, and not that quick in use. It is easier to use a faster film when you need the extra stop. I have also found the 75 a little soft.
Hello Justin, I'm surprised at your comment regarding the 75 1.4 Summilux. >>I have also found the 75 a little soft.<< Are you sure it isn't your RF? I used that lens for a number of assignments not to mention quite a number of other Leica photographers I know who use it extensively and the last thing any of us would say is it was soft.
If anything it's too damn sharp! But shooting wide open @ 1.4 takes some major skill to make sure you are right on the mark as the depth of field is so shallow it's very easy to "be slightly off" therefore creating a slightly "soft looking frame." And due to using it wide open the backgrounds are a major blur also adding to an "appearance it's not sharp."
I'm not questioning your ablity to handle your gear, as I'm talking from experince and that of many others users. To many of us this is one maximum quality mind blowing lens. And costly. ted
I am confident it is not the focusing. One of my test objects is a picket fence which I photograph (on Linhof 4x5 tripod) at 45 degrees so that even if the range-finder is very slightly out somewhere will be in focus.
Rather than use the word soft I should probably been more descriptive and said low contrast at full aperture. The lens is sharp as you rightly say, however a full aperture which is what I wanted it for, I found the image a bit "flat".
At medium apertures it is a very good lens, but I don't need such a bulky lens for anything other than f/1.4 or f/2.