BCLK. It's the man's way to go about it.
In all seriousness, I would use a mix of both. Having a second variable to play with will often help get you the best overclock. Starting with raising the CPU Multi to get a vague idea of the extent of the CPU's potential headroom would be my first action. Based on the max clockspeed you manage, I would drop the CPU multi and raise the BCLK and see how far that takes you. Every time you experiment different CPU Multi's, BCLK's and Memory frequencies, make a note of the system's performance by means of some application (say SuperPi 1M maybe or if you want to do something more heavy, CinebenchR10) and once you've exhausted all of the maximum feasible settings, pick the sweetspot out of the bunch and start working on making it 24/7 stable.