Thanks for the welcome, people.
Tim,
The D300 gives me a number of options over the D70. The viewfinder is larger and brighter. More focus points (51 versus 5) makes for better control and composition and I regularly switch focus points underwater. I'm finding I get better colour rendition and my pictures look sharper with the D300 (I don't use in camera sharpening or picture boosting). The LCD on the back of the camera is much bigger. I'm far sighted (wear glasses for reading only) and don't wear a prescription mask. The larger viewfinder helps (a bit) for assessing shots underwater). Finally, and I hate to admit this, the extra pixels is a big bonus.
I get more value out of the larger viewfinder above water. Ikelite, in their wisdom, appear to have used the same viewfinder port on the D300 housing as the D70 housing. This means I still can't see the full frame through the viewfinder in one go. I got used to this with the D70 so it isn't too bad.
I have more here (for when you're not at work
):
D300