Stimulation. The G-spot vibrator gives internal stimulation, focused on the front wall. The rabbit gives internal and external stimulation at the same time. For many people the clitoris is what leads to orgasm, and then the question is whether you want the toy or yourself to take care of it.
Fit. A G-spot vibrator only has to fit internally. A rabbit has to fit in two places at once: the shaft internally and the arm against the clitoris. The distance between the vaginal opening and the clitoris varies between people, so a rabbit that's perfect for one person can be unusable for another.
Freedom of movement. A G-spot vibrator can be moved in and out, angled and turned freely. A rabbit can be moved too, but then the arm easily loses contact with the clitoris; most people hold it fairly still and rock it gently.
Controls. A simple G-spot vibrator has one motor. Rabbits often have two, and ideally they can be controlled separately. But there are G-spot vibrators with two motors too – We-Vibe Rave 2 has one motor towards the G-spot area and one towards the vaginal opening, controlled independently of each other.
Size and weight. Rabbits are bigger and heavier because they have an extra arm. Womanizer gives 185 grams for the rabbit-style Blend and Fun Factory 230 grams for Miss Bi, while LELO gives 93 grams for the G-spot vibrator Liv 3.