At one point, I shot with both a D300 and D700. They're still great cameras, but are definitely getting long in the tooth.
When I upgraded cameras I had a similar choice to make, but the DX choices at the time were not nearly as compelling as a D500.
If you're anything like me I think you can answer the question yourself with one simple question - the same question I asked myself when I upgraded:
When you go out to shoot, which do you reach for most often, your D300 and D700?