Well the obvious advantages of the DX50 over the X3 are:
1 - touchscreen interface, which makes it easier for the user to scroll through lots of music. doing that with physical buttons can be a bit tedious.
2 - better screen quality - it having ips technology is a big plus in sunlight...
3 - better layout of physical buttons - much more intuitive compared to the "slanted" style on the x3, not to mention dedicated volume buttons (even this one is a deciding factor for me personally)
4 - usb OTG function which should at least be able to connect to NAND Flash drives (be it usb thumb drives or external ssd) or if the power supplied is good enough - to the external HDD, which to be honest, is a really big plus for some.
5 - replaceable batteries - this in my opinion, is a pretty good move. I've read James fiios opinion on replacable batteries in the X5 thread and he said that power banks are way to go. for me personally, I'd rather carry an external storage and tiny pre-charged batteries, rather than a power bank AND external storage... YMMV...
6 - I'm not entirely sure about this one and I apologize in advance if this turns out to not be true, but the AMP section should be more powerful on DX50 compared to the X3.
we can only speculate on the rest until someone has both and does an exhaustive comparison...
I don't have any of them and I like both, but as of right now, I would choose DX50 over the X3.