since i am new to mi2s, which custom rom is best for me now?


For you? No idea... Since you're new then why playing with custom roms? Just stick with MIUI.
 
http://xiaomi.eu/community/threads/rom-marvin-english-multilanguage-xiaomi-mi2-s-17-9.20996/

Excellent rom (the english one, multilanguage has bugs). Can't remember when was the last time I charged my phone :p And battery life is just one of the many aspects that are improved. Read the first post for all the tweaks and customizations marvin has added.

But if you're new you should do some research about cwm, flashing etc on MI2/s before moving on to a custom rom.