Not to bring back the debate but just to point something out. They can be almost the same thing sometimes too.
Three of my GWP's grandparents are DDs. Some better GWP breeders stick to a very high standard and breed very close to the DD lines. The problem with GWPs is that the bottom of the barrel is much lower then that of the DDs. At the top there is likely very little difference. Show and pet bred GWPs are not going to be a DD.
My dog Deuce is a slick, I actually seem to see more photos of slick DDs then GWPs.
As far as being aggressive...if you mean towards people then nope. Mine sure doesn't show that. He will bark at someone walking by but that is because he thinks they should come pet him.
If you meant towards fur...well mine doesn't kill cats but I wouldn't tempt him too much. I know other dogs of other breeds that are not cat friendly. He picks up very quickly on what I will let him hunt. He only once tracked a bumped deer and will usually only sniff out a rabbit for a few seconds before moving on. Not hard to train them to 'Leave it'.
If you mean towards other dogs...nope, and in a big way nope. It will likely get him hurt some day but he trusts other dogs more then he should. He is so friendly and happy to see other dogs that he doesn't think they could be mean.
The one sort of negative thing I would say about Deuce is that he is not for the casual couple time a year hunter. He drives me nuts if we aren't hunting or training at least a couple times a week. He's not quite 2 so maybe that will tone down a bit but from what I have heard from other owners this is just how some of them are. He is a driven pheasant finding fool and that's what I wanted.
Tim