On Heroku I was using thin that forced me to scale horizontally to handle more than one request at the same time. With this setup I can set a number of unicorn workers bigger than 1 and I'm done. I can scale vertically until I can and then set up more machines with the same methodology and use a load balancer like HAProxy to manage requests.
There is a limited amount of workers you can run with 512mb of memory (max 3 for my application) and the random routing can overload one dyno while the others are doing nothing. Basically you have to scale and pay more when with Digital Ocean I can buy a 2gb machine and run 10 workers spending 20$ per month.
