the 500 was not changed much since 1989 althought it was sold untill 2002 in the U.K and 2004 in the U.S. i have to be carfull here because there are some serious fans of the k5 on here. the k5 i had was an 89 so was pretty bad to ride (through old age) and combined with the fact there was little devolopement of it between 89 and 2004. compared to a modern mx bike the k5 is a tough beast to handle. so the advantages, basically are 10 plus years of development. the kx 250 chassis it lighter narrower with supirior frame geometry and suspention for motocross which is the what this bike will be used for. thats not to say the kx500 is a bad bike because its not, from all account it is an exellent dessert bike too.
having said all that neither bike would be suitable for a bigginer