What are you trying to solve with the text captcha? Users might get confused what is going on there. If a spammer wanted to create a zillion accounts, they could very very quickly write code to solve a captcha like that.

Those kinds of captchas work well for blog comments where the spammer is trying to get their link on lots of separate blogs, but usually spammers aren't trying to create accounts on every free service out there. They either want a zillion accounts on your service or they don't care anyway.