John, I'm sorry to tell you but I have no experience with the a850 or the a900. Anything I say about comparing the two would be just anecdotal. I have been using a Sony a77 and would say that the a99 is two or three stops better with noise than the a77. The high ISO files are crisper and more detailed at high ISO (3200-6400) than the Canon 5Dmk2 I used to shoot with as well.
My only Sony experiences are in this year and with current product. I'm shooting a corporate show all week. Much of it will be available light. I'll try to remember to shoot some relevant samples for you and post them here or on my blog.
The EVF is an important part of the process for me.