Reasonable on 05 May 2010
The 360 has a lot of people paying to be able to play online, and online is a core element of the console, so online games selling better wouldn't surprise me.
However, I wouldn't say that SP games are underwhelming on 360, as many have performed well globally. I'd say that SP/MP titles simply have a better chance of performing really well on 360 (although good WRPGs do sell well on the console, too).
The fact is MP is popular, games with MP sell very well on PS3, too.
I rarely play MP myself (I find it boring and repetitive ironically enough) but I can plainly see what is popular right now (and has been for a while).
Try to be reasonable... its easier than you think...
