My question is which of the following I should invest in...
(a) a 3.1 GHz quad-core Intel core i5 iMac with 16 GB memory??
(b) a 3.7 GHz quad-core Intel Xeon E5 Mac Pro desktop with 12 GB 1866MHz DDR3 ECC memory??
Firstly, I edit with Premiere Pro CC.
I shoot on a DSLR platform and am looking at investing, also, in one of these Atomos monitors that record raw / ProRes (?) signals from something like a Canon 5D Mk III.
I plan to shoot the occasional 4K video using a GoPro camera, but would immediately crop/convert it for edit and output to 1080p. I plan to also use my GoPro to shoot 1080p video at up to 120 fps.
I do light 2D effects with After Effects (mainly titles and green screen), but no 3D rendering whatsoever.
Output would be no greater than 1080p.
Lastly, I would need to connect two monitors and an external drive with FW800 or USB 3.0.
Is the Mac Pro required for the work described above, or could I get away with the iMac? Any advice is greatly appreciated!
An iMac is going to handle that just fine, be it an i5 or an i7. I'd opt for a little bit more RAM (24Gb or the 32GB maximum).
You're looking at a 27-inch model, correct? Those still allow you to add more RAM via a small door in the back. If you're looking at the 21.5-inch, you'll need to buy it with the desired amount of RAM from the start.
You'll be able to connect a second display via a Thunderbolt/DisplayPort Mini adapter (I usually buy mine from monoprice.com). If you go with a Thunderbolt display, you can daisy chain two Thunderbolt displays via one Thunderbolt port (although, Apple discontinued their Thunderbolt display). If you go with a non-Thunderbolt display, you can connect two but they'll each take up a Thunderbolt port and there are only two to begin with.
Here are the Apple specs for connecting displays: About Thunderbolt ports and displays - Apple Support
You're good on USB3 ports with the iMac and you'll need a Thunderbolt to FireWire800 adapter from Apple.
The upside of the MacPro tower (if it won't break you budget) is that it'll do everything that the iMac will do, but faster; however, for what you've described, the iMac should be a good machine.
Great - thanks for the advice, Warren!