Yep, MS SQL Server. I think that's something you have to determine on a case-by-case basis. If you have Software Assurance you can run it on any instance but if you don't you have to pay Amazon for dedicated hardware. As usual Microsoft's enterprise licensing is pretty much shit. You might also incur more operations overhead than you can afford by having to deal with patching, outages, performance degradation, etc. And your management might want to engage in a slide deck circle jerk with animations about how modern and in the cloud you are by using RDS so that's worth throwing a few $30K+SA enterprise proc licenses away.