I have a MacBook Pro and 2011 iMac, the one just before the switch to the new ultra thin model, and both are fantastic performers.
Unless you are going to do heavy duty graphics or calculations that produce a lot of heat, the iMac is great. If you are going to do things like 3-D gaming and a huge amount of rendering or ray tracing, then the Mac Pro would be better because of the cooling capacity. The Mac Pro does have the upgrade advantage since you can add more and more memory, four hard drive bays, change the graphics card. Which means greater longevity.
But it really comes down to the fine details of what are you going to use the machine for and how far do you really think you will go in placing demands on the system. For the brief items you gave initially, the iMac would in all liklelihood be teh best solution. And with the 27" you can upgrade the memory if you find the base 4 GB isn't enough. Memory is about half the price Apple charges when you go to suppliers like OWC and Crucial.