Well while submitting my clients site to directories i just found this topic to discuss on. I had submitted many sites to many directories and found that most are using alphanumeric captcha. Being directory submitter i like numeric captchas only as they are easy to type. Let me know your opinion on this.
I have done a lot of submissions and it bugs me to no end to have to guess when letters are involved. Go with numbers only is my suggestion. It will save both you and your submitters a truck load of grief and bother...
One major disadvantage of having a numeric-only captcha means the 'manual' (or semi automatic) spammers' job becomes even easier and thus, you are much more likely to receive spam with an easy captcha. But on the other hand - you may also scare away customers. Walk quietly but carry a big stick is probably the best choice