It has five times the killing power and tissue penetration of doxycycline.
It is a protein synthesis inhibitor.
Metronidazole is typically sold as Flagyl.
It is a smaller molecule size than advanced generation 5-nitromidazoles.
It therefore has better tissue penetration.
It can have worse side effects.
It is used to bust up lyme in cyst form.
Lyme can become resistant to both of these and have to be changed farely frequently.
Brian Rosner talks about this in his book "Top Ten Lyme Disease Treatments".