If I'm understanding your question correctly, then, and I'm just somewhat repeating what you've already stated, it comes down to whether or not you want the boosts on your accounts that don't have the Epic version. If you want the boosts, then buy the Heroic version. On the other hand, if you wish to skip the $20 boost offer, then buy the Base version, but the option to grab the cheap boost from upgrading may still be available in the future. They've offered the digital upgrade in the past, but a boost has never been locked behind a particular version before.

Also, I'd be careful if you're buying anywhere other than Blizzard right now, since RMT is becoming rampant as stand-still bot farms try to squeeze out as much as they can before Shadowlands hits and nerfs all those open-world farms into the ground.