I think it depends on what you're doing. If you're primarily sending material, like a news service or support, the logo is fine. It's a great branding tool. If you're interacting, the face of a human is more preferable, imho.
Think about it from your potential audience's perspective. I usually don't follow accounts with a non-human name and definitely don't follow those with a salesy name. The relevance of their tweets is the next criterion for me. If self-promotion exceeds the rest, I don't follow. The actual avatar is the last of my considerations. Most brands I follow show their logos, as they have more than one person tweeting on their behalf. Self-employed professionals usually have their own photos, which works for me. In this group, someone using a non-photo avatar tends to irritate me.
Given the dubious quality of internet anonymity, there's little chance that I will be posting pix of myself online anytime soon. I'm less concerned with advancing my ego than I am with preserving my hide.
I use logos for corporate accounts and photos for private accounts. Find a good one and stick with it. When you're following a lot of people it's best to be consistent with the image so that when you show up in peoples' streams they have a chance of remembering you.