The new Sigmas promise better QC but the optical formula of the 17-70mm is, as far as I know, unchanged. So, what you'll be getting should be something akin to a golden sample of the previous version.
The 30mm f/1.4 and 17-70mm lenses have different uses and advantages so a comparison doesn't mean much. Just be aware that the zoom won't match the low-light ability or thin depth of field of the 30mm. If, as you say, you've got a good copy of the 30mm, I'd recommend you hang on to it if you possibly can.