Hi Romeomaxx,
As you can see from the responses so far, different programs vary in their ability to make use of multiple cores. If you're using a program like Logic in its latest incarnation, then more cores is great, if not, then you're better off with higher speed and fewer cores.
If you're looking at the low end, which I am guessing, since you're comparing it to the iMacs, then I would look at maybe the Apple refurb store, or the 2009 machines from B & H...
http://www.bhphotovideo.com/c/product/608729-REG/Apple_MB871LL_A_Mac_Pro_Desktop_Computer.html
Those are going to be far better machines for sound and video work than an iMac - they will hold more hard drives, which are easily replaceable, can handle far more memory, and are far more upgradeable in the future. With that in mind, the best iMac for those purposes would be the most expensive one. :/
Go with the Mac Pro, even if you have to get one of the lowest end ones. Reading on these forums, you'd think that everyone needs the most expensive machine every day, but unless you're an extremely high end user who is doing cutting edge work, the difference between even the cheapest Mac Pro and the most expensive isn't that huge. Things like having enough memory, using SSDs and spending your money strategically to get the most bang for the buck make a much bigger difference than you'd think getting all your information here.
For the record, I am a lighting designer, photographer, and video guy, and I do a little sound design on the side, and my 2008 Mac Pro is still a very sweet machine that I feel no need to replace. Spending Five Grand on a new Mac Pro would make my videos render a little faster, true, but generally, if I'm waiting on a video to render, I'm going to let it do that while I sleep or go out and do something else, not sit there and watch it. In that context it doesn't matter to me whether it renders in 2 hours, or 2.5 hours.
So yea, price out the most expensive iMac, then go spend that money on the best Mac Pro you can afford, leaving yourself enough cash to buy at least 12GB or 16GB of RAM and at least one OWC SSD. You'll be happy with that machine for years.