What is the main difference between V2 and v3 ? Is there a way to use V3 cloth to V3 ? Or maybe to use V3 morphs and textures on V2 ?

V3 is the newer model. She's more resource-intensive, at least when fully loaded. (V3 has injectable morphs, so you can inject only the ones you are using.)

V3 morphs and textures will not work on V2. However, you can use V2 textures and clothes on V3...sort of. V3 comes with a bonus V3-to-V2 figure, which is basically V3's head on V2's body. You can use V2 textures and clothing on it. And probably body morphs, too. You must use V3's head morphs.

V2 is cheaper, and might be preferred if you have an older computer or if you just hate the injectable morph system. V3 is the state of the art figure, though, and new products tend to be all for her. Very few people make stuff for V1/2 any more.