Choosing the right AWS instance

Hi everyone,
I am generally interested in a procedure to choose the right instance to run Stan models on Amazon AWS. I am using Louis Aslett’s RStudio AMI and want to run RStan on RStudio Server.

My model is quite complex (Hidden Markov Model where the transition probabilities are driven by time-varying covariates). I ran it on my computer (2 cores with 2.00 GhZ each) and 1/15 of my final dataset. It consumes ~ 1GB RAM and ~ 50% of CPU. I am wondering which assumptions I can make to make the right choice for the AWS instance out of all the instances listed here. And how would the computational effort increase if I want to run a 3-state model instead of a 2-state model?