Depth on a pitching staff (anywhere actually) is not a bad thing. Especially when you are talking about adding veterans with experience to a mix that will have some young arms along with some pitchers that are coming back from injury. Ironically, both Oswalt and Wilson themselves are coming back from injuries to one degree or another. Realistically, I dont see the Mets signing both of them, and If I had to choose only one, I would lean more towards Wilson.
When Oswalt was last pitching, we was relatively ineffective. Wilson was pretty much dominating until his injury. Wilson will still need some time to be at 100% and I would not rush that process, so he does not relapse. This will give the Pen an opportunity to find its pecking order based on opportunity and results that are already in house.
Parnell will compete and help push Francisco and the more reliable pitcher will be the closer (the other the 8 inning man). Once Wilson is ready, he can compete with the current closer, and give the most deserving candidate the closer role, and the other gets the 8th inning or 7th inning specialist. A lot will also depend on lefty/righty match ups and the success (or lack of), by Feliciano, Edgin, Carson, Mejia, Byrdak ETC.