I would go with the SRM450's myself. Each SRM450 has 400W RMS of power while the PMX2000 has a 'total' of 500W. With 2 SRM 450's that gives you a total of "800W" RMS output power. Plus you'll have less gear to lug around too. The Behringer PMX2000 plus passive speakers would be a less expensive option though. The PMX plus 2 Mackie C300's would be a few hundred less than 2 SRM450's but like I said, you would be lugging more gear around. That's a good enough reason in itself to go with the SRM450's.

Best regards,
Mike