I had a 6400/6401 and now have a 361. Stock, I think the 6400 will outcut it. There is a weight penalty, but both are great saws. Bucking firewood, the 6400's extra weight wouldn't be noticed much.
Now, a point that seems to be seldom raised about the 5100. The 5100 uses a different bar mount that the 64/73/7900. The Stihl 260/361/440/441/460/660 all use the same bar mount. If you add another saw (what are the odds on an AS member adding another saw), if you stay with Stihl, you can reuse bars. Just something to consider.
Oh, you will buy lots more saws. You understand that, right?
joat