I have £1500 to spend... Upgrade an old Mac Pro or buy a new iMac?

I work at an agency where we are increasingly working on more video based projects. These include editing HD videos and 2D animations / motion graphics. We don't regularly work with 4K footage. We don't work on 3D animations either although some of our 2D animations utilise 3D layers.

We mainly use Adobe After Effects and Premiere Pro.

I have a budget of around £1,500 to spend and I would like to know what would be best to do... Upgrade an old Mac Pro with new hardware top be used solely for video editing or to buy a refurbished iMac 27-inch?

The specification of the old Mac Pro is below:

Mac Pro Mid-2012

3.33 GHz 6-Core Intel Xeon

14GB 1066 MHz DDR3 ECC

ATI Radeon HD 5770 1024mb

1TB Hard Drive

OS X El Capitan

I am not the most clued up when it comes to the hardware side of things so if it is best to upgrade the old Mac Pro, it would be really useful to know what some good upgrade components will be.

Any input on this will be much appreciated.

Thank you.