I've had a SX-15 for years before I finally got my VE 40" cutter... talk about an upgrade. Anyway, for the SX-15 you should use Dr. Stika as it is the native cutting program for the Stikas... however, it is proprietary and anything you create in the program will be forever unreadable by other programs. Sadly, all of the customers' graphics that I've created over the past two years are stika format only...
Anyway, if you have any questions regarding the stika or Dr. Stika, let me know. I've been doing wholesale runs of 5000+ 6" graphics on my SX-15 and it's still running great.
Matthew
BlackIceFX