Mouse = Skill. Get the best mouse to get the best skillz!
<Comments on the MX518. No idea about the G500, although it seems to share some DNA>.
I'm not a fan of variable DPI. Always stays max on the MX518. I also prefer simplicity, so the 518 suits me fine. Plus, it feels rugged. It's very much like my MX510, with higher variable DPI. The 510 is still working perfectly after so many years I've lot count.
I've had a Roccat Kone, which felt a lot more fragile (the weight compartment can't hold weights anymore, the scroll wheel is very flimsy), but more complex / tweakable. Had some minor issues with the drivers, no such problems with Logitech. So I went back to what I know and trust, a MX518.
The 518 has 5 usable buttons, it's not many. Two thumb, and the standard 3 (left right, scrollwheel). The DPI buttons works, but I never use them. I can't remember if you can reconfigure them or not. The scrollwheel button is stiff, but the others are good, responsive and have a nice 'click'. The feel exactly like on the MX510, so I'd expect them to last forever.
The MX518 shell looks a bit 'funny'. I'd rather they'd have gone with a more austere look (maybe some light show), then it would have been perfect. In truth it doesn't bother much me really.
It is a big mouse and slightly on the heavy side. That's something to consider if you prefer small light mouse.
my $0.02.