I think that Marines are more 'technical' with their range of skills...Paras seem to be more of the 'hard as nails mad bugger' type shock troops....both superb.
I read in a book about the Falklands. that when the Marines entered Stanley, they passed a group of Argentine prisoners under guard of 2 Para... The paras barked an order, and all the prisoners stood up, and gave the Marines a wanker gesture as they passed ;-)