D50 or D70 how to choose & portrait lens?

I'm new to DSLR - know some of the basics and have used manual SLR in the past. I have a Panasonic FZ5 which is not SLR, but want to move into this area. Hoping it will give me clearer shots? And faster to catch kids.

How should I decided between 50/70? Being a somewhat amateur?

What is the real difference? Like to get into casual portraiture - what's the best lense for this?

thanks!

KHK