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.
It really depends on how you want to be perceived. I personally think that using a face is a good idea just because it seems a bit easier to connect with people on a personal level.
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.
by my point of view you should use your pic, it helps get familiar with other users so, they can follow you.
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.
I will prefer Company Logo as the profile picture, it will promote your Company Logo also.
A logo would be beneficial if you are using twitter for official use. If the account is personal and bears your name, then it should have your pic.
Logo will be perfect if it really is more than a branding image, it needs to perfectly depict what is your business
so at the first glance users will know ahead of time what to expect.