All of them are generalized to the point of useless, but also have a grain of truth to them.
Generally speaking you don't really say 'love' until you're pretty damn serious about someone as a cultural idiom. Ai Shiteru is pretty formal as a phrase and not said lightly or in jest in normal conversation, at least not when sober.
Suki da is generally good enough for simply dating up until things get really serious as far as general purpose. Some people translate is as "I like you" but it really is stronger than that as it's used in the context of dating.
Of course, as with any culture, there are always those who are way too willing to just jump in and act disgustingly saccharine with each other early on in a relationship, but due to the formal nature of Japan it can often make one party uncomfortable if the other is too forward too fast. Not necessarily enough to break up, but enough to make them apply the breaks and embarrass them even if it is only said in private.
I would say "Ai Shiteru" is harder to get most Japanese people to say over getting an English speaker to say "I love you", but that has more to do with the fact that "Suki Da" exists as a phrase, and English speaking people use "I love you" to express both ideas with one phrase. "I love you" is also used informally in western cultures as an expression and not in a romantic context frequently, and you don't hear "Ai Shiteru" being used that way in Japanese.