I have the Vr6 and the 6.5. The clit arms are not the strongest I've ever felt, but they've got a bit of rumbliness to them (most people will say that same thing, that the clit arms could be stronger). I also just got the Vr7...I guess it's basically the Vr15 but without a flippy thing on the clit arm. The Vr7's clit arm compared to the Vr6 & 6.5, well, it feels a bit stronger power-wise, but it is buzzier than the other two.
If you want internal and external vibes, I would suggest the Vr6 or 6.5 because most people say that those two are probably the strongest out of all the smaller rabbits. But if you want a rotating shaft with a vibrating clit arm, go with the Vr15, or even the 7 if you are like me and don't like those flippy little things.