You should pick stmoehing that inspires you. Also you should take into consideration just what your going to do with your sword and dance. Will it involve floor work? Will you be balancing it a lot? Probably you want a piece that has a medium or slow pace as this matches better with the slower floor work moves and balancing. However, if your looking to show off a lot of sharp hip snaps while balancing you might want a medium or fast paced song. You may also want to consider what style of bellydance you are preforming. Is it a nightclub style? ATS? Fusion? etc Then try to pick music that matches better with that style. Many people prefer to do a sword piece with music that is more ethereal, new age-ish or mystical in sound things like songs from the beats antique cd's which are pretty popular. Watch some sword dances on youtube and see if you like the feel of anything they are dancing to.