The prices seem to vary greatly.
which is better ?
I have seen the following.

Sony Vegas Movie Studio 8.0 Video Editing Software NEW on eBay, also, Computer Recording Software, Pro Audio Equipment, Musical Instruments (end time 14-Sep-08 15:52:13 BST) (http://cgi.ebay.co.uk/Sony-Vegas-Movie-Studio-8-0-Video-Editing-Software-NEW_W0QQitemZ290253513484QQcmdZViewItem?hash=item290253513484&_trkparms=39%3A1%7C66%3A2%7C65%3A16%7C240%3A1318&_trksid=p3286.c0.m14)

SONY VEGAS PRO 8 + DVD PROFESSIONAL VIDEO EDITING on eBay, also Editing, Video Software, Software, Computers Networking (end time 03-Sep-08 08:30:41 BST) (http://cgi.ebay.co.uk/SONY-VEGAS-PRO-8-DVD-PROFESSIONAL-VIDEO-EDITING_W0QQitemZ280261285501QQcmdZViewItem?hash=item280261285501&_trkparms=39%3A1%7C66%3A2%7C65%3A16%7C240%3A1318&_trksid=p3286.c0.m14)

NEW AUTHENTIC SONY VEGAS PRO 8 VIDEO SOFTWARE NEW! on eBay, also Editing, Video Software, Software, Computers Networking (end time 03-Sep-08 21:21:54 BST) (http://cgi.ebay.co.uk/NEW-AUTHENTIC-SONY-VEGAS-PRO-8-VIDEO-SOFTWARE-NEW_W0QQitemZ310079277563QQcmdZViewItem?hash=item310079277563&_trkparms=39%3A1%7C66%3A2%7C65%3A16%7C240%3A1318&_trksid=p3286.c0.m14)

Vegas Pro 8 is the current full featured version. Vegas Movie Studio Platinum is a subset of the Pro version (some features are not there). Vegas Movie Studio is a subset of the Movie Studio Platinum version. So it all depends on whether or not the cheaper versions have all the features you need. Otherwise, go for the full/Pro version.

If you need to author blu-ray DVDs, you'll need the Pro version with DVD Architect 5. The current Movie Studio Platinum version can edit HDV and burn blu-ray straight from the timeline but not author.

FYI - Movie Studio 9 is the latest version in the studio series.