Welcome to FT. While I'm not familiar with RDU, the conventional wisdom is that you need plenty of padding when you're connecting flights that are booked on separate tickets, because if you miss the connecting flight, the airline will tell you tough luck, and you'll have to buy a new ticket. Presuming you'll have an internet connection while on the plane and in immigration, I'd book the Southwest flight knowing that their cancellation policy is relatively generous. But frankly, neither flight makes me particularly comfortable. I'm relatively risk averse, so my strategy would be to book at 8 or 9 pm flight and hope I could go standby on an earlier one.