I'm going to say A.
Technically speaking, I think I'm somewhere in between A and B honestly. I do like the 30 day period because that kind of forces me to focus and move a little quicker than I normally would. I do want to try out the 100 day scenario just to see how it works out. And, given the fact that I haven't actually completed either project of the last two 30 day challenges, I think 100 days does give me a better change of having something mostly complete. Thanks to my work schedule its sometimes a little hard to stay on topic for the 30 day ones, but given 100 days I can completely drift away from the project, do something else, and then come back when I get excited about it again. (Or look things over, say "this is crap", overhaul the design and take off again) Those things kind of happen with me sometimes and I should concentrate on having a project closer to the finish line by the end.
Take care all, and good luck with your en-devours!