I'm not familiar with the Magnums,but i've owend the 500 and now have the 1000,and as much as i liked the 500 the 1000 are in my opinion,clearly superior,so if you don't intend to buy the 1000,do yourself a favor and don't listen to them,these things are addictive.
As for the differences between the 500 and 1000,i'd say that both have good bass extension,altough the 1000 is a bit better controlled,the mids on the 500 are on the dark side wich gives them a certain warmth, wich altough unnatural,can sound appealing to some,the 1000 mids are very accurate and remind me of the RS1i in that regard,the treble on the 500 is not as extended or airy as the 1000,and last but not least,the soundstage of the 1000 is wider and deeper than the 500.
I still believe that the SR-80i and the PS-500 are THE best buys among the current Grado line,but there's a reason why the 1000 costs more than the 500,it's a superior headphone,now,as for the price difference,only you'll know if it's worth it,and regarding the comment of a previous poster about having money to burn,i don't agree,if there wouldn't be any significant differences between the two,than that comment would be justified,but i can assure you that this is not the case.