Depends what you want to do. If you need f/1.8 get the 50mm. If you need a wider angle lens get the 40mm. If you need the smallest lens possible then 40mm is your lens. Sorry if that sounds simplistic but that's really what it boils down to for the most part.
More nitpicky aspects: the 40mm will be slightly sharper over the whole frame at f/2.8 if slrgear.com is to be believed, and the 40mm will focus more quietly, and if you have a newer camera body more accurately as well. The 40mm is also better built than the 50mm which has the nickname "plastic fantastic" for a reason.
Thanks for reply. i just wanna get some pics in party hall or auditorium in low light