Me?, I prefer the middle and don't care for a window. When I'm on Conquest or Ecstasy, I'll usually know the M'd and request a table, but it's just up to the computer or luck the rest of the time. I will always take a round table if possible-just seems to work better.
You can always change on the first day, but I'm always second guessing myself wondering if I just changed from a great group of guests on a table to a bad set of guests just for a better location.
We too prefer the lower level, center of the dining room at one of the big round tables for 8. We've also enjoyed having one of the long tables near the aft windows, but conversation is much more difficult at those tables. Like Snooze says, it's pretty much the luck of the draw.
I agree with the round table, but there are exceptions. The very best table we ever had, oddly enough, was a small table for 4 in a corner. Somehow we were matched with a couple that we bonded with immediately. There were a couple of times, for both couples, that we were really too tired to go to the MDR but just HAD to find out what the others had seen or done that day. I guess it is really more about the people than the shape of the table.