For taking to the track "someday?"
A roll cage is ridiculously over the top, and unnecessary, and immediately changes your street car to a track only car. The cost of a roll bar increases your level of safety, but again, maybe a bit much for AX, and especially for your first few days of HPDE, if this is your first time tracking?
I went with a bolt in DAS bar after my first season of HPDE, and POC's Performance Driving Series. My bar was a direct bolt in, and took about 30 minutes to install, after I removed both seats. Cost with shipping (across country) was around $1400, of which $500 was for shipping. This bar could also have been welded in. Welding is a much better way of securing the installation, but obviously much more permanent. For you though, unbolting and removal is a much easier process if you decide you don't wish to slide down the slippery slope!
I know that Dave @ TRE in SoCal, also makes a bolt in setup that is very high quality.
Ready for some instruction? Come join me at Porsche Owner's Club (POC) Performance Driver's Series Driver's Clinic #1 next weekend! We still have a few spots open, if you're interested!
http://www.porscheclub.com/events/EventCalendar/SE154-Info.html
http://forums.pelicanparts.com/porsche-autocross-track-racing/656167-porsche-owners-club-pds-clinic.html