I have never used Olympus SLRs so I have no comments on Olympus SLRs.
The choice will be strictly based on what you wanna use your camera for.
However I have used Nikon film SLRs and Digital SLRs. I would not choose Nikon D50 because it does not offer me many advanced features as D70s does. If you are not going to use advanced features offered by D70s I would not suggest anyone buy D70s becasue it will gonna cost you extra bucks. D50 is for entry level photographers and D70s is for mid-level to advanced users.
I own D70s and I love it. If you are an advanced user, want an extra resolution and are read to spend a thousand dollar more (than D70s), go for Nikon D200.
Here is an image of my collection of lens I shot with my D70s DSLR.
