If you have a hosted order form with Chargify, there's no difference between OpenGateway and Chargify when it comes to PCI compliance. VISA and MasterCard say you should be PCI compliant either way.

Also, OpenGateway doesn't store credit card information which is always good.

it depends on your sales volume. If you're a small starup you can do self-certification. Once your volume picks up you can go to the next step.