There are differences - such as no depth preview on the D50 and some differences in commander modes for the flash. I have two friends with D50s. I have a D70. Their pictures look as good as mine. It might be wise to spend the price difference on a better lens and wait until you are ready for a D80 - I have one of those also, or a D200.
I believe the D50 and the D70 are identical meters and sensors but calibrated a little differently. I don't think it's worth the extra money (I believe it's $100 difference?). Less of a difference then the D50 to the D40.
It's worth it. The D70 has better AF, DOF preview, a backlight for the top display, and most importantly TWO WHEELS so split aperture and shutter speed control
I use a D70 and the few differences are worth it to me. I use wireless flash all the time, the 2 dials are key, it's a larger, more robust body, also i use the DOF preview.