It's really a matter of preference, but two-factor is better security than two-step, and shouldn't be dismissed lightly (not that you are, but just saying for all) ... I think Apple should be pressured to roll out two-factor at a (much) greater pace than it currently is.
To understand really why it's better, an explanation into the terminology. Two-factor means that two of the three factors of information are used to validate identity: What you know, what you have, and what you are. A password is what you know. A trusted device (phone, etc) is what you have. A fingerprint or retina scan is what you are. A trusted device can be an authenticator or key fob, basically something you must have possession of.
Two-step is lesser security insofar as - for example - I can know my wife's information (what you know) and answer her password and security questions, and I'm in. If she had two-factor installed, the devices she has would be used to authenticate, and I couldn't use her account without also having a device AND knowing the information.
Apple will use the devices themselves as trusted authentication mechanisms - obviously behind keycode / fingerprint scan - which qualifies for the "what you have" element of the two-factor.
Now really, lets not split hairs here: I agree that it's probably just as hard to get your password AND THEN get your security questions. If were a nefarious person and had your password, without two factor I could try to determine your security question responses and hope you weren't reading your email (Apple does send notification of logging into services from new locations, which is good). If you're not monitoring email however, I could keep bashing away at your answers, and hope they were simple. Or do it ad nauseum in hopes you weren't really watching your account notifications. This is bad because unless you're watching mail, you could miss this.
On the other hand, if you had two-factor, all your trusted devices would immediately be notified of a new login attempt, and you'd be aware of one factor of your authentication being overrun and you could fix it safely and immediately.
Sorry for the long response but this is mostly for Apple and everyone else -- yes two-step is decent security but two-factor is preferred. I am among the many who are waiting (impatiently) for my account to be able to go to two-factor.
And Apple, if you're listening, please make it so you can see if your account can go to two-factor without having to disable two-step ... that's really not a good thing. Currently the only way you can enable two-factor is by disabling two-step. Backwards to go forwards... it would be nice to at least have some sort of notification on appleid.apple.com (upon logging in of course) that your account has the viability for two-factor.