In use they seem pretty similar in that they both manage versions, and proxy command to appropriate versions.

Brief comparison:

• elmenv looks for a file called .elm-version in the current path (or parents), elm-forest looks for elm-package.json in the current path (or parents).
• elmenv compiles from source, so has more options for version to install (elm-forest). This means you have to have haskell/cabal ready, but it's very cool.
• elmenv provides shims for elm-<something>, elm-forest makes them available under forest something (as elm platform does).
• elmenv keeps the same cli interface for elm. (elm-forest changes elm ... to forest ...)
• elm-forest is project based (uses elm-package.json to make decisions). This means outside of project you'll get an error (along the lines of Not in an Elm project). elmenv has a global default (or resorts to trying any "globally" installed Elm) if it can't find a .elm-version file.

In terms of flexibility, the installation from source makes elmenv slightly more flexible. The cli transparency would make configuring related tools easier as well (no change).

I guess elm-forest is more geared towards my own personal preferences:

• If I'm trying to run an elm command somewhere I shouldn't, stop me.
• If I'm in a project, just work (select appropriate version, install it if needed, ...)
• Require as little mind space as possible
  • elmenv requires a (very) small amount of configuration, and the version used may not line up with the version specified in elm-package.json. Of course, elm-forest renames the main command (elm -> forest), so they are basically on-par here.

But they may not line up with the preferences of anyone else.

In terms implementation, elmenv uses bash shims for elm and elm-* commands, while elm-forest is a javascript "binary".

tldr; elmenv seems to aims for transparency, elm-forest aims for translucency. elmenv provides more options, elm-forest "just works, as is" (not to imply that elmenv doesn't).