dilema

i have as the title says a dilema i want to buy all the seasons of charmed on dvd but i don't know if i should buy each 1 of all 8 one at a time which they are bout 22 dollars each or go for the whole set which is 150 dollars heres the catch my sister owns the first 6 seasons on dvd but it is very important to me to get the myself and "own" them literally call me a bit stubburn but yeah random thoughts on this from anon people maybe?