im on a tiiight budget here..
The MX518 is $50 and the G5 is $70
as far as i can tell, the only difference (ones that would matter to me) are the sensors... ones optical and ones laser.

Ive looked at the other specs like dpi, acceleration, image processing etc...
they both seem highly responsive.. i believe both are 16bit ..

anyway. in the end, im deciding between optical and laser, like.. what so good about laser compared to the optical MX518? Id really like to save the $20 for sometin else

U can get optical and save the cash but the optical won't work on all surfaces and they both won't work on glass. So the laser would be better in that sense but its very pricey/ Why not try this one its wireless and its from Logitech too. Plus, its way cheaper than those two. Give it a try.
http://www.newegg.com/Product/Produc...82E16826104196

The 518 has an extra thumb-button that I miss with the G5. The G5 has also lost the "switch between progams" button, but since I never used it I don't miss it...
The placement of the Increase/Decrease sensitivity are also better placed on the MX518, since you can easily reach both. Now they're both "above" the wheel, ie. closer to your palm, which makes them harder to use, especially in a fire-fight.
On the other hand you'll get a 4-ways wheel with the G5, but how often do you need that.
The biggest difference, that really does make a difference, is that with the G5 you can adjust the weight of the mouse, either as a whole, or more to one side or the other. And this is quite nice. And you get a maximum of 2k dpi instead of 1.6k dpi with the mx518. But since I haven't been able to test each mouse against each ohter (they're on different continents) I can't say how much difference that makes...
Hope this feedback can be useful...

I went to check the mice yesterday, but the G5 didnt have the weights on the display model.. so without them, its a tad bit lighter then the MX518 which i like.. I also like how the G5 does not use the same glossy top which could get slippery when my palms sweat a little.. im not a sweater so i dont think it will matter much. they both conform to my hand really well.. I think my hand is perfect size for these mice

I read around abour the 1600 vs 2000 dpi.. seems like its a matter of how fast these mice can go I dont think i need 2000dpi.. Id prolly be using the 400dpi option most of the time anyway.

As for the buttons, i dont think it will bother me much. ive never used any logitech mouse before... ever ... so either way, it'll be a new experience