I thought the were fairly equal when it came to strength and how strong they were. Buffy had more experience, patience and discipline so that certainly gave her the upper hand. Until Faith can learn those skills, my guess is Buffy will usually beat her in any head to head fight.
As far as emotional strength, my votes goes to Buffy; although I thought both were strong. Buffy was much more stable and able to cope with difficulties in a more mature manner; but Faith has been through a lot in her past, and the fact that she was able to put her life back together speaks volumes. Faith seemed a little immature, especially when it came to relationships, but my guess is as she gets older and gains more experience she will become emotionally stronger.
Mentally speaking, I'm not sure. I'd say Buffy was more mature mentally, but I don't know that I'd say she was more intelligent. Buffy certainly used her head more when making decisions, but I don't think that necessarily makes Faith less mentally strong. Faith seemed totally capable of making rational intelligent decisions, she just chose not too; which speaks more to her selfishness and immaturity than anything else.