Jay-D is a DIY DJ mixtable and Synthia is a DIY digital sampler. Both are music kits you build yourself, and they do different jobs.
Jay-D has a display. Once it is built you can play more than one music file at a time, fade between songs, control the BPM, design your own light show and make your own songs and sound effects.
Synthia has no display, but it does have a built-in microphone, so it can record. It combines a sampler, a drum machine, a sequencer and a synthesiser, so you can record your own voice or any sound around you and build beats out of it.
Both cover the same ground electronically: digital-to-analogue conversion, microcontrollers, digital audio, sound waves, LEDs and the individual electronic components and how they work together in a circuit.
If you are choosing between them, pick Jay-D if mixing and playing back tracks is the appeal, and Synthia if recording and making your own sounds is. Message us here, or email [email protected], if you would like a hand deciding.