The Dot and Tap serve different functions. Tap is great to use as a portable speaker but you have to push the button every time you want to activate Alexa.
The Dot does not have a built-in battery, so it isn't portable, unless you use an external battery pack. What the Dot gives you is voice-activation of Alexa--which is huge. You can just say "Alexa, what's the weather?" or "Alexa, turn on the bedroom lights." from across the room and she will answer/follow your command.
The Dot also has the advantage of allowing you connect to external speakers, either via the audio output jack on the Dot or via Bluetooth.