I have not used an N3 yet...but in regards to something like headphones, it is a huge difference. From my understanding a regular Bluetooth stream is lossy compression (not sure the amount of compression) and Apt X is at 16/44.1.
Every current Bluetooth 'codec' (SBC, AAC, MP3, aptX, LDAC) uses some form of compression. Newer proprietary codecs like aptX, apt-HD and LDAC use less compression (and different algorithms) to achieve "better" sound quality than plain-vanilla SBC. Other codecs like AAC, when used with AAC files from iTunes or your iPhone, don't need to "recompress" the original files (although AAC files are already compressed). That's why headphones that support AAC (which at 256kbps is equivalent to CD-quality) can sound as good (or better, depending on the source files) than aptX, although aptX achieves CD-like quality with any CD-quality source file.
Back to the N3, Cayin gets it wrong showing an N3 wirelessly connected to a car stereo and claiming "lossless and wireless" (see below)**. aptX is still a lossy format, but it can sound much better than basic Bluetooth. This graphic also unfortunately hides the fact that while the N3 can receive Bluetooth (and therefore double as a "wireless" DAP), it can't receive aptX, so if you're streaming high-res Tidal, Spotify or Flac files to the N3 from your phone or computer (or even high-quality MP3/AAC files), they're going to first be "recompressed" over standard Bluetooth before they even get to the N3. How much of a difference you'll actually hear using a car stereo is debatable, especially since the N3 has great circuitry that will deliver decent sound even over standard Bluetooth. Will it sound as good as aptX-compressed Bluetooth? Probably not. It won't sound as good as lossless streaming over Wi-Fi (which you can't do with the N3), and definitely not as good as hardwired (line-out) from the N3 to the stereo (which is the best option if sound quality is your priority).
No doubt having bi-directional Bluetooth is a great feature, especially at this price point, and many (more expensive) DAPs don't have any sort of wireless features, let alone aptX support. Still, would have been nice to have bi-directional aptX, although I'm holding out some hope that Cayin can add AAC and/or MP3 codec support in firmware, which will at least improve the sound quality of AAC/MP3 files sent to the N3 from our phones.
**Well they got it half-right - it is wireless (just not lossless).