What details do you want to learn ? basic linux sysadmin for web deployment ? Cloud Management ?
Linode is a traditional VPS provider, will give to you a Linux system that you can update, install components, etc
Amazon EC2 is a little more complicated for setup than Linode but will provide with much more technologies to be explored (EBS for persistant storage, S3 for alternative storage and serving static content, etc)
Heroku is based in Amazon EC2, maybe you can opt for some basic plan in Amazon (suggestion, at least buy some EBS storage too) to begin exploring Amazon services (exist very nice Rails plugins/gems to use Amazon services ...)
Another alternative can be RackSpace Cloud, in esence, they reconvert a clasic VPS service (SliceHost) in a On Demand computing service, maybe more simpler than EC2.