Neither D-50 nor JD-800 have to sound 'dated' if you program them from the raw oscs upwards. It's all in what kind of patch you make (or tired old preset you use) that makes or breaks it.
D-50 has PWM, JD doesn't - don't think anyone mentioned that.
JD-800 has a 'better' filter, in many ways much much better - however it's also a bit more sterile.
D-50 has live waveform generation (in structs 1/2) which is 90% of what I used my (now on sixth!) d-50 for. In this mode nothing else sounds like it, it's beyond warm for a digital and sounds pretty amazing (and not dated at all). D-50 has the slight edge in tone, but JD-800 has the edge in immediate power i.e the filter.
They are really quite different synths. The V-synth in many ways continues from where the D-50 left off (samples + 'VA' mixing if you wish) while JD-800 was only samples.
These two synths are two of my favourite ever (as is the V-synth), and the D-50 definitely has more character, more warmth, more thickness, more mojo/magic - it's a crazy little thing with a beautiful sound if you stay away from the 80s chiffs, panpipes and all that famous pop stuff from 87-90. It's a 'proper' synthesizer, just like DX7 is and like analogs are. It doesn't have to have any PCM involved at all (and you CAN have 4 oscs of pure synth/non pcm - you can have 4 oscs of whatever you wish - polyphony is still plenty).
The PWM adds a bit more magic again, the quirky filter which isn't like anything else is also part of the character, as is the mild hiss
The keybed is one of the best roland ever made (also used on the Alpha Juno 2) and is much better than the one on the JD-800. It's sleek, beautiful, and does things NO other synth can, inc software, not quite the same way.
It's a classic and 5/5 from me (for contrast I'd give the DX7 4/5 and the m1 1/5 lol).
JD-800 - Now, this is more a choice about the interface and the cool filter. The sound quality from this machine is a lot higher than the D-50, more pristine, 48k PCM short samples used as building blocks - that's very high quality for 1991! The interface is of course a beauty, the synth is extremely impressive and fun looking, it's not quite as sexy as the D-50 due to it's sheer size, but instead is just very impressive and cool looking. The hands on is fun and allows not just much easier editing but real time tweaking without any stepping (super smooth/resistant sliders).
Sound wise it's more static than the D-50 and is definitely, firmly digital. However the filter kinda makes up for it as it's smooth and can be quite aggressive and analog like. You can make some very cool epic pads and sci fi textures, punchy basses, weird fx. I think your ears will pick out it's digital nature faster than with the D-50 in 'synth mode' (non pcm).
It's really quite hard to compare as while you can get the JD to kinda emulate some D-50 stuff, they can both do things the other can not. And the D-50 has a more complex engine when you looking under the hood, with the aforementioned ability to combined two oscs within a tone and work some magic on them, while JD is a straight ahead 4 separate layers/synths (pros and cons to each engine really). So D50 has 6 LFOs and JD has 8 - but per sound/osc JD has only 2 while D-50 has access to 3 (though shared with it's other partial) so you can see tradeoffs all round.
This is why you should get both and see which makes the best sounds for your ears. I often find it hard to chose between them, when I want clean filter tricks and epic lows and scary sounds, I'll go to JD-800 (as D-50 doesn't/can't do 'scary'), but If I want a sweet thick analog style lead that sounds just as good/better than some analogs then I'll go D-50. It's early 'va' engine somehow sounds nothing like VA (In a good way) and retains character often missing in modern digitals. I don't know how they made that chip inside sound so good...
Anyway D-50 is closer in engine to something like a JX-10, and JD-800 is a newer breed.
sorry if anyone got bored part way through, I could ramble on about Roland's digital flagships (D-50, JD-800, V-Synth) all night.
Ultimately my choice is V-synth from the 3, yet I still own the other two. Roland made great digitals.