The fact that it's Magento shouldn't change anything, assuming you're not on bargain-basement hosting. And DPM, while somewhat more complicated to integrate, has the advantage of no exposure to credit card data. The only potential downside is it's not set up well for international addresses - there's no address 2 field, for instance - and since it submits directly to Authorize.net, your options are rather limited when it comes to dealing with that. You can kludge your way around it using some rather messy Javascript, but to be honest, I'd use a different method or a different payment processor if you're likely to have a lot of orders outside the US.
