518/558/598 share the same drivers and sound profile. the differences lie in build quality (increases with price), comfort (increases with price) and that there are very small differences in signature (more bassy to more neutral as you increase in price). the hd518 can be called a budget hd598 if you can deal with the build quality.
the 558 includes upgraded pads over the 518. if the price is that steep you could just buy pads yourself and save money. build quality is also improved b ut if you are careful its not a big deal. sound signature will be very similar and more than a 90% match.
usa price is $65 vs $99 which makes it more palatable and the 558 a worthwhile choice. if its really $70 difference where you live it may not be...