If you read previous pages on the D50, there's some issues:
- DAC is bad (some even saying it's terrible) and that it is better to use outboard DAC.
- D50 gets quite hot.
I love the design, it's just beautiful... The size is also pretty good. I'd actually love to own one, it's in my wishlist...
However in 353X you have space for another STAX earspeaker if that's something you'd use.
Also I think I've seen mods for 353X, not so sure about the D50 also due to size might be a bit more crowded inside the casing to upgrade components or apply mods.
I'm just new to all this, so take it with a pinch of salt and maybe looks around for these issues. The D50 is definitely not a non-brainer, so read around HeadFi, Reddit maybe ASR.