Depends if your going to use the six extra buttons or not. I use both; palm and claw grip and both mouse are perfect for me. Abyssus is longer and a bit bigger around but for medium sized hand user? I'm going to say both because I have big hands, and G300 is perfect for the hybrid grip I have going on. But it also comes down to feattures, and what your going to use it more of. Try both of them and see which one you like. Hope this helps.
~I've been using mouses since they game out (Abyssus 3.5G to be honest) and I love them both~ <3