One Big Server: Scaling Up
Many Small Servers: Scaling Out
Some say scaling vertically for up and scaling horizontally for out.
It is a business question more than anything.
Scaling your infrastructure up is fast, cheaper, and easy. You just hit the next size on your droplet. The downside is that you don't have any redundancy.
Scaling your infrastructure out is much more complex. You need a load balancer and you have to think of a lot more technical questions like session management and caching. You get redundancy too.
The correct answer to your question is a combination of redundancy needs and how much time and money you want to spend.
The general idea of using a service that provides droplets is that you can focus on growing your business and just scale up until you get into the higher levels. By the time you grow into the high end levels your business should have enough traction that scaling is a "good problem to have" and you can afford to put some investment into it.
So to answer your question: For now you scale up. When you get near the limits of scaling up you invest in scaling out.