Do some research on the CP4 pump if you want a diesel Powerstroke or Duramax. If you keep your vehicles a decent amount of time it could be an issue, for you to decide how important. If you trade frequently it likely won't be an issue. The Cummins doesn't use that injection pump.
As far as gas versus diesel goes, if you can afford more up front, the overall cost is pretty close to a wash. So my advice is make that call based on factors other than cost - it's more about personal preference and performance.