About 8 months ago, i replaced my MX500 with a MX518 gaming mice, so i was in the same situation as you are today.
I took the Mx518, because i heard that some former logitechusers weren't satisfied with their Razor models, and because the G5 had just one thumb-button back then.
As conclusion i can say that the Mx518 is a nice mouse.
It's light, slides quite well, the mousewheel is good (more silent than my old Mx500), and the configable DPI-values are very handy in some situations.
Handlingwise i couldn't realise any big differences.
Well, the MX518 is abit lighter, but overall it feels like any other MX5xx mouse in your hand.
In my opinion the only negative point of this mouse is the material of the holographic surface.
It isn't rough enough and feels abit too slippery. Due that material i've to clean this surface every 3 days, because you'll notice any tiny piece of dirt on it.
Some comments even mentioned that this material causes sweaty hands. Until yet i couldn't notice that.
After you spent some years with a MX510, i advice you to buy whether a Mx518 or the new G5 refresh.
Which one? hmm, difficult question.
If you don't mind to spent 20 bucks extra, i would choose the G5 refresh.
Not because it has some DPIs more (you can't notice that anyway), only because it has an other surface and lasertechnology.
if you don't care about lasers and the slippy surface, go for the Mx518.
Also you can change sensitivity on the fly in-game, great for switching around depending on the weapon you wield (like for sniper put it way down so its more accurate).
same goes for the MX518.
With the SetPoint software you can enter profiles for games aswell, so that's even possible with a Mx518.
do you have 2 buttons on the left side like on the 510/518? I read a review saying they took it off, but some pictures show it with both?!
There are two versions of the G5 out there.
The first version had just one thumb button. After logitech realised that people want to have two thumb buttons they made a second edition, called G5 refresh.
Compared to the new G5 refresh (the one with two thumbbuttons!) the MX518 is cheaper and performs almost the same.
The G5 series uses Lasertechnology, the MX5xx series is on optical technology.
Lasers are superior, because it can handle more surfaces, but tbh i never had problems with my 518 yet.