I have a 17inch BMC SuperStroke FS mtb. What kind of rear derailleur should I get.?

I mean Long, short or medium cage. Is there a "standard" or any indications that would lead me to an answer. It has 34-11 tooth cette and regular chain rings. Will it shift Sh*tty if I have the wrong cage? I have a XT long cage Rr derailleur on it now.