http://www.audioholics.com/techtips/setup/loudspeakers/hometheaterspeakerlayout.php
6.1 is just like 5.1 or 7.1 except you put the rear center speaker directly behind you like a 7.1 speaker, except not to the side.
Sound isn't a huge jump overall with 6.1 vs. 5.1.
BUT: That is not really the important thing.
The speakers on the Onkyo are much larger than the Yamaha - much more significant and will result in a much better range. The Onkyo also has a much better A/V receiver at the core which gives you better power and more inputs & outputs to utilize.
Read the specs at Onkyo:
http://www.us.onkyo.com/model.cfm?m=HT-S770&p=f&class=Systems
I don't call something a great deal without meaning it. It isn't the 'best' product in the world, but I think at that price point, it is by far the best product for the money.
On the link I gave - right about $200.00 shipped which is a total steal for that much of a system.