You should on GCE, but not on EC2: per-hour billing for EC2 means your 15 minute VM costs you 60 (aka 4x as much). We (sadly) have a 10 minute minimum, but the difference is pretty stark.
Once they got through some growing pains (rate limits per project), Travis CI has switched to a 1 VM per build model that they'll probably detail in future blog posts.
https://blog.travis-ci.com/2016-04-07-migration-update
Disclaimer: I work on GCE, so of course I want you to use it ;).