Given the description of your layout and your listening habits, the M60s will definitely be enough speaker for you -- 95% of the time. It's that extra 5% of the time that you need to consider when choosing between the M80s and the M60s. The primary difference between the M60 and the M80 is that the M80s play cleaner and louder and can handle ungodly amounts of power. The times when you feel like saying "To hell with reasonable listening levels! I want to hear my music really loudly in the bathroom on the other end of the house!" you will be glad you have the M80s.
Soon after purchasing the M80s, you'll want to upgrade to a receiver that can handle a 4-Ohm load without having to flip a switch. Such switches limit the power output of the receiver to prevent it from overloading. It's a big compromise just so the manufacturer can claim their product can drive a 4-Ohm load.
