Focus on breath has always been good at relaxing me and clearing my mind a bit, but I've always found that a focus on my heart (usually while placing one or both of my hands directly over the area of my heart, on my chest) better grounds me (slips me out of ego-consciousness).

I'm not sure why this is. It might just be that it's easier for me to let go of thoughts for attention on my heart vs. on my breath. Or maybe there's something more to it?