For updating an  Android OS which one is better amongst the, official Update and Custom ROM update
According to Sony's official statement, Ice Cream Sandwich (a.k.a. "ICS", a.k.a. Android 4.0) is coming to Xperia Arc at end of April or beginning of March of 2012.  If you are worried about the cons of rooting (i.e. voiding warranty, possibly breaking the handset, etc.) I would suggest that you wait for the official update from Sony and/or your carrier.