As of right now I'd go with Welker also, although Welker hurt me at playoff time. Matthews history of injuries. I would be concerned is of him going back to Matthews of years past. I had him a couple of years ago and he did little for me.
I think it is a wash and it depends on need. Welker is the safer play, but Mathews could be significantly more valuable if the Chargers go run heavy again and he is able to stay healthy.