I've used a range of different instance types for 5 and 3 phase calculations. The five-phase method is easily parrallelized (using the -n option in rcontrib), so I tend to use one with lots of CPUs. If you're doing an illuminance calculation I'd recommend using the compute optimized instances. If you're doing annual renderings I'd suggest using a memory optimized instance and copy the matrix files to the instance's SSD drives.

I almost exclusively use the spot instances, which risks having your simulation shut down mid-way, but usually results in a 70-90% savings on rates. With a little experience, you learn which instance types are likely to spike and when. For example weekends and holidays are a good time for spot instances.

Also, I recommend using the alarm to shut down your instance when the CPU drops below 0.5%. But make sure you're putting your results onto a network attached drive, as the instance's drives are lost when they're released. I also highly recommend using Amazon's EFS service for storage.