Math, Understand your concern for your friend. The big Honda is NOT for everyone as are most big thumper dirt bikes. The 650 is a much refined and improved version of the 600 but most of the basic caracteristics still remain. Your friend needs to seriously consider what's most important in his ride. The 650 is most certainly on the next dimension from you friends 400. I personally chose the 650 because I love having the ultimate in big dog torque and power for mostly playing. My experience has been there are very few who can challenge the 650 with a decent rider on hill climbs speed burst etc. Anyway the 650 with all its improvements over the years is still heavy and very exhausting on endurance length hare scrambles, motorcrossing etc., usually starts hard after crashes and even can be embarrasing trying to load or unload the big pig off of one of these mile high new trucks.