Rival has way worse materials/build quality than the MX518, IMO/IME. and it's not really comfortable for me, whereas I find the MX518 to be really comfy.
honestly, I really like the G400s, if you can get ahold of one of those, it'd be a straight upgrade from your MX518 in every way. though it doesn't look much better, IMO
and some will argue that due to its sensor implementation, it is a little bit less responsive than some of the other options, but I didn't find it to be a problem. it seems to be a matter of a few miliseconds. can't even say if it's faster or slower than the MX518.
the G502 is probably the best hardware you will get right now, but the scroll wheel isn't really an upgrade to the MX518.
I would also like to recommend the Roccat Kone Pure Military, as the gripping style is fairly similar to the MX500-shape, but smaller.
but since you're considering the Rival, you also have the option of the Zowie EC1 eVo CL if you use a good solid-coloured mousepad.