Logitech G300 - Two buttons on the side, left and right, and two in the middle under the wheel. Also, the laser is a bit off center; not a big problem or anything. Changes different colors such as red, blue, green, white, yellow, ect.
Abyssus - Bigger for palm grip/claw style hybrid (like me) but perfect for palm grip or those with longer fingers like myself. Blue LED light, right and left click with a normal scroll up and down wheel that clicks. You can also switch the DPI and respone down under neath the mouse.
I took my Logitech G300 back because I wasn't feeling it for MMO's, RPG's, and FPS. I play FPS more, and MMO's was nice but I just wasn't using it as much like I should of. RPG's; never gotten a chance to play it in Skyrim.
Abyssus on the other hand can go up to 3500 DPI (if you get the 3.5G version) other than that, G300 beats the Abyssus by 700 DPI making Abyssus 1800 DPI and G300 2500 DPI.
You can also use the driver to change the sens in it but you can have to sign up on Razer website. Logitech? You just download the drivers but doesn't have firm update. You can also choose if your right or left handed using the Abyssus.
All in all? Both are great mouse and the Abyssus is just a simple gaming mouse which I love. If you want simple, then that's the mouse for you.
But if you know your going to use the six extra buttons then go with the G300. I love both mouse but for playing a lot of FPS, Abyssus is right for me. <3