You are rtalking about two very different machines. The iMac 27" can be upgraded in memory but nothing else. The Fusion Drive is very, very fast but you might want to consider the 3 TB over the 1 TB if you are going to do a lot of photo work. And an external hard drive would be a big help for backing up for your own protection.
The Mac Pro is powerhouse and very expandable at a later date. As you fill up hard drive space, or just to have backup internally, you can easily just plug in more drive volume. Memory is easily upgradable as you find you may need more. The graphics card is just that, a card that can be replaced or upgraded. The downside is the Mac Pro has not been "refreshed" or revised in a few years. Many seem to believe it is overdue. No one knows what Apple has in mind, however, so no one can reliably say what or when changes will happen.
Just some thoughts.
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.
I had the MacBook and the iMac both purchased around the same time maybe 6 years ago. Both the hard drives crashed, the iMac I was told was completely fried. I love my apple computers and I think Im leaning towards the iMac 27' now. My sons recently started making videos and music on them so I wasn't sure if that was the cause or they're just old.
Thanks again for all the insight!