Evernote for Mac: I haven't upgraded to v5 yet (for the very same reasons you and everyone else have mentioned), so I'm running the latest pre-v5 release for Mac: "Version 3.3.0 (300201)"
Evernote for iPhone: When you upgrade an app, iTunes throws the old version in your Trash the next time you sync the phone. (Unless you did the upgrade while connected to iTunes, in which case it puts it in the Trash then.) I've never had to downgrade Evernote for iPhone, but I have succesfully done it with several other apps by following these steps:
If you haven't emptied your Trash since the day you upgraded, go there and dig the oldest version out. (There may be multiple versions... You want the one with the number closest to the current version.) It'll be a self-titled ".ipa" file. (If you have emptied your Trash, you'll need to find another copy somewhere else. Try Google.)
With iTunes off and the phone not connected, swap that file out with the new/active app version. (Usually located in: ~/Music/iTunes/Mobile Applications).
Manually remove the app (the newer version) from your phone, then connect and sync. iTunes should load the old app version, which is now in the active location.
Over the years I've learned to retain/pull-out old iPhone app files before emptying the Trash -- as a "just in case." It's come in handy at least 5-6 times.