I have had both and prefer the MX518. I hate the mouse wheel on the G5, it tends to move sideways when I don't want it to and doesn't feel as rigid as I would like. The MX518 has no side scroll but I never used it on the G5. I found the weight thing on the G5 to be useless. I'm also not a fan of the braided USB cable because it snags on the edge of my mousepad or on my desk.
I have owned a G9, G5, MX518, and Deathadder but I prefer the IME 3.0 with the IMO 1.1 being a close second. Deathadder would have been perfect for me were it not for the rubber coating. Sold the G9 and G5, gave the Deathadder to my brother, and I'm keeping the MX518 for when I get a 1920x1200 display.
What "I" would do in your case is try to find a cheaper MX518, I got the red one from someone here on the forums for $25 shipped. Otherwise, if what I said about the G5 doesn't bug you, it is an awesome mouse, you can't go wrong with it.