The Previous Systems folder just contains everything that was replaced or overwritten during the Archive & Install. How big it is will depend on which option you took. Basically, regardless of which option, it is not being *used* -- when you are comfortable it contains nothing you want, you can delete it. However, 15GB seems like an awfully large folder if it doesn't contain your old user folder. You may wish to see if iLife, etc still work -- since Garageband, iDVD, and iWeb all store large amounts (several gigs) of templates and samples in the ~/library folder. At worst, though, you'd just need to reinstall the respective app.
