depends on the printer and what you are looking to do with it. If your doing any print to cuts etc, I would go with flexi. Much easier to use. (It can be done in onyx but involves using Illustrator. If your ripping much larger files or don't have a need for print to cut, go with onyx.
Bottom line, both will do what you want to do. Onyx is a rip only program, while flexi lets you rip and output, and is also a damn good vector program for cutting vinyl and doing layouts etc.