Mortgage payments: semi-monthly or extra payments?

Which way is more cost effective for a mortgage payment: paying semi-monthly (400 fee to enroll) or making a extra payment of $ 500 /month towards the principal: i.e: 30 year fixed at 6% and paying $ 1500/month. Should I enroll on the semi monthly program or pay $ 2,000/month (1,500 + 500 towards principal every month)?
Thanks to all

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