I'm the author of the ethereumjs-testrpc, and did much of the work on Ethersim during its early days, evidenced by this commit, the following commits and this unmerged PR. Ethersim was started by Iuri Mathias as an alternative to eth-testrpc, also written primarily by me, adapted from a script by Stephan George.
Ethersim was woefully unfinished when I found it, not supporting enough requests for the unit tests mentioned above. After a few late nights I had completely rewritten Ethersim, keeping Iuri's file structure but rewriting the internals to better integrate with ethereumjs-vm. Iuri eventually accepted this PR, and Ethersim has been working against this rewrite ever since.
Continuing to make Ethersim better, I submitted this PR, mentioned above. This PR made block handling better, and included two special RPC requests (
evm_revert) which could significantly speed up unit tests and, if merged, would give Ethersim the ability snapshot the chain state at a specific block and revert back to that chain state later.
Unfortunately, this PR was never merged. As time went on, the ethereumjs community decided they wanted to consume one of these projects, either Ethersim
master or my changes in this new PR, as they were so closely tied to ethereumjs-vm. After discussing it, they chose to consume my fork as it seemed to have had more active development at the time.
Since this fork, their feature set has greatly diverged, with ethereumjs-testrpc consuming most if not all of Ethersim's features (I'm biased here, of course, but I highly encourage you to compare their README's). At the time of this writing, the following features exist in the TestRPC that do not exist in Ethersim:
- Snapshotting and reverting the chain state, speeding up unit tests significantly.
- Address creation with HD wallet support, useful for integration with other applications like Metamask
- Deterministic address creation, useful for debugging
- Optionally processing blocks on an interval instead of instamining, simulating a real blockchain
- Instantly forking from another currently running chain (i.e.,
geth), useful for a wide range of analysis and scripting activities
- Starting and stopping mining with
geth, also useful for scripting and unit tests
- Configurable gas price and gas limit
- Many more RPC requests supported
- Better defaults for simulating a real blockchain
As far as I can tell from the README, Ethersim does not have any features that the testrpc does not support.
So the TL;DR, as biased as I might sound, is that ethereumjs-testrpc is a more fully featured blockchain simulator and has a wider array of uses. They both have the same goals, but ethereumjs-testrpc has seen more active development.
EDIT: Somehow I missed Karl's answer before answering myself, not sure how. This at least gives a history of their development and why a fork occurred.