Cricket isn't played in stasis; the vicissitudes of form play an important part in any team's selection.
Personally as of today I'd have Broad (height and bounce plus swing when pitched up) & Philander (seam movement and nagging accuracy) as the first two seamers on the World XI's sheet. Steyn and Anderson are both more similar in their MOs, swing bowlers who typically bowl a full length. The former's quicker, the latter moves it both ways through the air. Despite Jimmeh's undoubted improvement, Steyn's figures, both overall and more recent, are better. So I'd go with my namesake.
It's true Philander has only played seven tests, but he's taken 51 wickets at a tick over 14 each. Leaving him out would be "bloke who rejected The Beatles" stuff.
& I'd pick Swann as the spinner on the reasonable assumption that the Martians have a more rigorous application of the laws pertaining to chucking.