Best Progression from a Viper Mk III for solo HazRes farming?

My current goal in E:D is to work my way up to a Federal Assault Ship. I tried out a HazRes last night in my Viper Mk III and had a blast - plenty of high bounties and as long as I managed my aggro by avoiding aggressing wings and focusing on singletons, very survivable.

I'd like to consider a ship upgrade to try and make the process faster, and I am currently debating on whether saving for an A-rated Vulture or Diamondback Scout would be the better option. The DBS seems cheaper and more maneuverable, with better damage output, but less tank, as far as I can tell.

What are people's recommendations?