Thanks for elaborating, that makes sense. Since I noticed in another one of your other comments that you're planning on a freemium model, I think Option A is a good enough starting point. Once you get the ball rolling, it would be interesting to test both options to see which actually converts better.
I may be the only person with this question, but if I use Twitter to sign up, will it have any impact on my timeline (like an automatic "I just signed up for Infogr.am" tweet)?