I happen to own and use both the D70 and 7070. Both are great cameras in their price class, but really very different classes. You can probably buy a 7070 with housing for $700-800, while a Nikon D70s with lenses, housing, ports, and strobes will cost around $3000-4000. The 7070 in an Olympus housing is compact, inexpensive, but much slower in operation than a D70 system. For a beginner, the D70 is bulky, and very complex to learn to use and maintain, and probably not a good choice.
The 7070 is a great camera to learn with. It has a great lens, and very good image quality, similar to the D70 in terms of resolution. The lens is especially good at close up images, since it will focus to just a few cm for small close subjects, like nudibranchs, and will shoot wide angle too. The camera is much slower in operation than the Nikon D70, taking 1/2 second or more to focus and set exposure after the shutter is pressed. If your subject is stationary, such as a nudibranch or frogfish, that is not a problem, but for rapidly moving fish, it becomes an issue.
The Nikon is more expensive and more complex to use. It's not really a good camera to start underwater photography with, but possibly one to grow into. You will have to plan each dive carefully, choosing the lens, port and flash setup for the subjects expected, as it does not have the built-in wide range zoom with close focusing capability that the 7070 lens has. For rapidly moving subjects, the D70 is a better choice because it has minimal shutter delay.
My suggestion is to go to a shop and operate both in their housings, to get a better feel for them. Frankly it is probably best to start with a simpler camera and learn with it, then see where your interests lead you. After 50-100 dives, you will have a much better idea what sort of camera fits you best, based on what you like to shoot and the results you have achieved with your first camera.
