Mostly depends on what cranks you have and how much cash you are willing to part with. With good quality cranks... Sq. Taper: Strong, plenty of space for decent bearings, cost effective to replace the BB. ISIS: Stronger, stiffer, these days you can get a very good BB with good bearings for around £35-40. With cheap cranks... Sq. Taper: SHOCKING. ISIS: Better option, they wont "round off".