Adobe and other software makers also seem to be moving now to OpenCL with the latest software so should be able to use the FirePro Cards in the various Apps nicely. Even on Windows Premier Pro is adding OpenCL support so Adobe products don't seem to be forcing you down an Nvidia route anymore.
Having said that it will probably require you to upgrade to the next cloud service version of the Adobe Suite rather then stop with v6.
In terms of speed then the Mac Pro as previewed with the right software will completely outclass even a fully maxxed out iMac. In terms of storage then both will be external unless you can live with an SSD and Single HD in the iMac.
Unless they drop the specs and have a simpler base version then I believe that many of the casual owners that bought the Mac Pro simply for the storage expansion are going to be priced out of the Mac Pro for the future.
Personally it is too rich for me, unless there are lower spec base versions confirmed.