A generation apart: Audioengine HD3 is the newer design, against a 2011 model. Newer is not automatically better, but the older one is more likely to be discounted or near run-out.
| Area | Better | Why |
|---|
| Bass | B | Audioengine A5+ digs deeper - 50Hz against 65Hz. |
| Built-in power | B | Audioengine A5+ has the bigger amplifier (50W vs 30W RMS). |
| Connections | A | Audioengine HD3 adds Bluetooth. |
| Size | depends | Audioengine HD3 is easier to place; Audioengine A5+ is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | Audioengine HD3: a sub output; Audioengine A5+: a sub output. |
| Price | A | Audioengine HD3 is cheaper - $349 against $399, about 13% less. |
A Audioengine HD3 B Audioengine A5+
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both speakers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
The usual trade-off: Audioengine A5+ measures better, Audioengine HD3 costs about about 14% more% less. Whether that gap is worth the money is the call only you can make - and in a normal room it is often smaller than the numbers suggest. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 65Hz anyway, which makes the cheaper Audioengine HD3 much easier to justify. In a large or open-plan room, Audioengine A5+ has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Audioengine HD3 is the easier speaker to get right.
And remember what a spec sheet cannot show: how each looks in your room and whether the finish suits it, what dealer support and warranty are like where you live, and how they actually sound to you. If you can hear them first - ideally with your own music - that will tell you more than any number above.