This is a really complex topic, without a clear answer.
Here's my understanding of things:
The TU-DST50 (also sometimes listed on the 'net and eBay as TU-DST50W -- so if you're doing a search try both) was the first generation model. The suggested list was $1499 US. One of the great things about it (as well as the 51) is that it decodes and outputs all ATSC digital TV formats,
including 720p (used by ABC); most HDTVs and set-tops downconvert 720 Progressive to 480p or 1080i. Some people feel strongly that 720p is superior to 1080i, so it's nice that the box can output both (esp. for owners of multi-scanning projectors or monitors).
The TU-DST50 can only receive and decode ATSC digital TV broadcasts (not regular NTSC/cable signals). It has a nice feature, though, in that it simultaneously downconverts HDTV to an NTSC signal and outputs it via an S-Video connection. This is nice if you're recording things (or checking recording status) with a PV-HD1000 and have a big CRT projector (a combination many forum members have) -- you can use a standard TV to check things and don't have to power up the "big beast".
Now, the TU-DST50 has a known problem: It runs very hot, and has some ICs in it that can't take the heat. Unless the owner has added some kind of after-market cooling, every factory-original TU-DST50 will eventually display a picture problem known as "Blue Sparklies" due to overheating. The word is, once the sparklies show up in the picture, it means the chips have been damaged and the problem is irreversible without a repair.
Here's where it gets complicated: Panasonic is aware of the problem, and will fix it (replace the defective ICs with new ones that are heat-resistent) -- but when they do, they also allegedly update the firmware to add 5C copy protection. Not nice. The best thing to do with a 50 is to get an original one that hasn't displayed the "blue sparklies" problem yet (and hasn't been "fixed"), and add some extra cooling (like a top-mounted fan) to it. This is very tough to find -- but brand new, "virgin" 50s have shown up on eBay from time to time.
The TU-DST51 (or TU-DST51A) was the second generation model, and it retailed for $999. The odd thing about it is that Panasonic improved it overall, but both added and took away features. It has an NTSC tuner built in, and can tune either digital ATSC channels or conventional cable and off-air channels.
Several people who have owned both units claim that the 51 has a better ATSC tuner, and is more stable and senstive to off-air HDTV signals.
The 51 doesn't, however, have the "simultaneous down-converted NTSC out via S-Video" feature. It does have a switch that lets you change its output to 480i NTSC, however (to set up or check a recording afterwards, etc. on a regular TV). The "simultaneous NTSC S-out" is the one feature I wish they'd kept (but they probably dropped it in order to add the NTSC tuner and lower the list price).
Most importantly, the 51 does not run nearly as hot at the 50, and does not have the "blue sparklie" problem.
Now, the story on the 51 and copy protection is murky: Some say it doesn't have 5C copy protection, some say it does. Some people (who seem to know what they're talking about) say Panasonic tried to add it when they released the unit, but that the 5C specification wasn't near finalized yet and the feature -- as implemented in the TU-DST51 -- simply doesn't work. The timing of the release of the 51 and the release of the 5C spec seem to back this up. I'd say that there is a good chance that the updated firmware on the 50 (when you send it in for repair) DOES work (since it is a lot more recent).
Anyway, it's a tough decision. There are pros and cons to both. I decided to go with the 51 because of the overheating problem on the 50 (just didn't want to deal with the potential hassle), and decided to take the chance that the copy protection doesn't/won't ever work on the 51 as implemented. It's interesting that, when the question is asked in AVSForum -- it seems that just as many people say the 51 is better as say the 50 is better. It all depends upon your requirements and priorities. I do know that the 51 seems to be more rare, more coveted, and commands a higher price on eBay. I know that some forum members use a 51 as their primary set-top -- but have a 50 mothballed (just in case of the 5C issue).
Anyway, this is my understanding of all of the issues; please, others jump in if I've left anything out or misunderstood any aspect of this (It's admittedly complex).