My advice - and answer to your question - is to pick a payment provider that has a reputation for stability, uptime and treating its merchants responsively when they need help... and to ask a couple of key questions to any prospective processor.
Namely, ask them all of the following:
1. Are foreign card transactions allowed? Or are they automatically scrubbed from certain areas? (the latter is true for a number of 3rd party processors and not the case with most true merchant account solutions)
2. What is the uptime on the payment gateway solution in question? (just like uptime is important for a server, the same is true for a gateway)
3. Does the merchant processor include any technologies that could be beneficial with your sales? (i.e. free shopping cart with affiliate tracking & reporting or other value-adds that might help you on the sales side)
4. How are risk management & chargeback issues addressed? Remember - all of the sales in the world won't matter if your funds are unreasonably held back (or never funded) due to a few chargebacks or other related issues.
All of the questions above have a direct impact on the sales that could be generated through the particular processor being interviewed.
However, it is worth re-iterating that a processing service doesn't really "make sales" but rather just "handles them." You still need to go out there and bring in the business in the first place.