What should I do about replacing the catalytic converter on my 1998 bmw 740i?

Can I go with an aftermarket like Eastern Catalytic, or do I have to go with original BMW part? This is the passenger side catalytic converter and the price difference between original and aftermarket is over $1,000.00.|||go for a used one with warrenty for 拢150.could last 2 months then again might last years.|||why do that if u can go to a junkyard?and gt it for like 100 bucks