Depends on how you're setting it up.

  aws s3 cp --recursive bunch-o-data s3://some-bucket/
  spark-ec2 --region eu-west-1 --identity-file s.pem --key-pair=spark --instance-type m3.2xlarge --slaves 40 -v 1.5.2 launch my-cluster

Is significantly easier than making PG work easily at the 100GB scale in my experience. spark-ec2 is a script that ships with Spark to make it easy to set up a cluster in AWS.