One feat or another

[ynnswa]

This cannot be as difficult as I'm making it.

Here is the Class Special Ability I'm trying to create:

At 6th level, the nomad gains Diehard as a bonus feat. If he already has Diehard, he instead gains Toughness.

My searches through the forum have turned up a couple things that are a bit similar, but I can't seem to adapt them successfully. I thought that perhaps the best route would be to bootstrap both feats to the Special and then handle selection in the bootstrap conditions, but I don't know enough about writing containerreq statements to know if/how it would work.

Any thoughts would be appreciated.