do yourself a favor, please look at Headfonia's comparison between all the HD5XX series. one of the most common mis-conception, and i wish people would stop recommanding fones based on this, is that there're a lot of modifications out there that will try to convince you that modding a HD5XX will turn it into a HD5XX. this is simply NOT true, and have been pointed out in countless reviews and A/B comparison. the sonic signature of each of these models are very different, even if they all share similar driver, but Sennheiser went into detail and did their homework and went as far as alternating the designs to improve the sonic signature. again, a modded HD5XX will NOT turn it into a higher model fone, it'll just simply be a modded HD5XX.
having said that, if where you live a AD700 is priced the same as a HD598, definitely get a HD598. because it's just THAT much better. in your case, i would say the HD598 is underpriced over there, which is really good deal if you can get it for $150. i got mine for $199 and that was with a huge discounted coupon.
comfort wise, again, having owned both. AD700 will make you think you're not wearing anything on your head (but if you have a very small head, that might be an issue), HD598 will be a very comfortable fit, but i wouldn't rate it right up there with AD700's comfort level. even that, i have no problem listening to music straight for 8 hours on my HD598.
whether a fone is warm or bright is a very subjective point of view. everyone has a different standard for brightness. but in general, i think all can agree that HD598 is the brighter can of the HD5XX bunch, even brighter and livelier than HD650. but if you're coming from AKG K series, the Senns will definitely sound relatively warmer to you. but consider the Senns warm signature, HD598 is definitely on the brighter and more energetic side.
and one last point, i highly recommand you try on a set of open vs closed cans. if you're still comparing M50 with the 598, sounds like you're still not sure on whether you want open or closed. in general i would suggest you narrow down Open VS Closed first, then go from there and find the models you like. since open cans and closed cans are very different animals.
again, look at headfonia's HD5XXX review, it'll tell you all you need to know about all the 5 series. the 518 is on the low end spectrum of the 5 series delivering most of the low end but that's all. on the highest end of the 598 you'll gain enormous soundstage with solid bass and great mid. the choice is yours.