6D is weather sealed. 60D isn't.
It isn't as simple as that - sealing is not a binary value - its Rebels < 60D < 6D < 5D3 < 1D ... I don't know how the 7d does in comparison to the 6d. The 60d has some sealing (but not at the lens mount point, so with an L lens it's only one side), but I shot in light rain with no problems. The added sealing on the 6d wouldn't make me confident enough to actually shoot in prolonged rain, and even 5d3 users broke their camera doing that - so water "sealing" is more for the peace of mind, for actual "safety" you'll need a 1d body.
Thus dust sealing is more important to me, and in this category even the 60d should do ok (unless non-sealed lenses, esp. zooms which will attract dust and sand like a magnet).