Well, if the options were to shell out $100-150 for on office license or pay $6 a month, which do you prefer?
After 18-24 months, you'd be at break-even, so then buying might make more sense over the long haul... except that when that new version of Office comes out you are shelling out another $100-150 for that.
If you will always want the current version available, then it doesn't make a lot of difference-- if you are the guy currently using Office 2003, then I guess buying a full license is probably the better value.