If you ask me, it's important to understand the gear you have, which comes very well from using it a lot. I use a six year old 30D with the 50mm f/1.4 (great lens, great choice!) and with the understanding I have of this gear from using it so much, I can very reliably take pictures that fulfill my vision. The biggest advantage I'd get in stills from a better camera is better low light performance, and the 60D is a low light dream. You can get a better body to fill in some particulars, but for the purpose of taking good pictures you already have a lot of what you need.
The 7D is like a 60D with weather sealing, no articulating screen and better autofocus. The 5D Mark II has better (?) weather sealing and the higher res, full frame sensor. Unless you need to get wider or have actually needed to make prints bigger than what you can do with 18mp, you wouldn't need that. The 5D III has...better low light I think? And that metering issue. I don't even remember what they changed about it.
If you have money to burn, get more lenses. People notice the difference a lens makes, but most people don't look at a picture and pick out how the minute qualities of a nicer body would have improved it. You might, but as long as the pictures are good, you've still got everything you need.
Although the others are right; if you're getting paid, get a backup body.
