In my area, residential is a lot more profitable per hour. We have a few nice, profitable commercial accounts, but most of the commercial stuff that comes across my desk I can't do because I would make less $ per hour, per crew, per day, etc. than I do with residential.
Of course, this depends on what you are doing. We get a very high hourly rate because the lawns in this area are pretty small. So I'll have one 2-man crew doing 20-25 lawns a day, all in the same neighborhood, and that brings me in $2000 - $2500 per month. If I sent that same crew to do a commercial account for the day, we'd only get about $1200 - $1500 per month.
Part of the problem, too, is that the commercial industry in my area is more competative. There are some big companies that use REALLY cheap labor and can lowball me to death.
Then there are other issues to consider. For instance, if 3 of my residential accounts cancel, it doesn't hurt too bad. But if 3 commercial accounts cancel, that hurts! Then again, it's takes a lot less time to get 1 commercial account than it does to get 20 or 25 residential accounts in the same neighborhood. So that's the trade-off I think.