The 20mm f/1.7 can collect more light than the 14-45. It's also sharper. The 14-45 is a 3x zoom. The 20mm f/1.7 is also much smaller, and the 14-45 is much larger. As you can see, it's a tradeoff. You might want both.
Personally I would choose the 20mm f/1.7, an ultra wide angle, and a telephoto, and just forget the 14-45, but that's me.
If the GF1 is your only camera, then the 14-45mm will be more useful because the zoom will allow you to frame your shots. If you use the GH1 as a travel camera or a party camera, then I'd recommend the 20mm b/c it's better for low light and better for portability.
BTW, you can use any micro 4/3rds lens on the GF1. You're not limited to the 20mm and the 14-45mm. You can also use pretty much any other lens on the GF1 with the appropriate adapter.