I think this is an interesting one... I guess it depends where you see yourself going with your service in the future. If you want to add more on to the server then getting a semi-dedicated now would prevent a migration in the future. Alternatively, the semi-dedicated would provide a faster service for your current sites and if that is important then head that way.... Sorry to be vague but I think it depends a lot on where you want to go in the future and your current
VPS Hosting spec.