Both are good guns, and both will need some work right out of the box, if you want to bring them up to their full potential. The B50 has a better stock and trigger, and I believe can be tuned for more power, if that's your thing.
The Library has "The Book", a multi-page PDF on all the things you can/need to do to the B50/51. It's a very simple rifle to work on, but like all things Chinese, QC is a crap shoot. I got one from a forum member that knows his way around an airgun, he was convinced it was cursed, it had an air leak he never could figure out. Once I got it, I stumbled across a binding valve stem bushing, problem solved. But it's the sort of thing that never should have made it past QC. Most of the basic fixes are simple and cheap, and there's only 3 or 4 of them. If you don't want/like to work on your air guns, you can also buy one pre-tuned from Big Ed the B50 guy:
http://www.bigedmachining.com/ By all accounts, he does magic with those rifles.
I've shot a Disco a few times, but don't own one, and probably won't. It felt toy like to me, and had a lousy trigger. That said, it was plenty accurate, seemed to have reasonable power, and is made in the USA. It's available with a pump for under $400 IIRC. Lots of mods and aftermarket parts available, and they are also available pre-tuned from several sources, although the only one I can think of off hand is Mac1:
http://www.mac1airgun.com/ Best bet is to call Tim and talk to him, he's not so good about email :-).
I don't know your budget, but for only a little more than either of these will cost, you can get a Marauder. It has a lot more features than either of the two you're looking at, and is a multi shot. If power is your thing and noise isn't an issue, the Evanix Renegade and AR6 are very good guns, but they suck air like crazy. Gotta get that power somewhere :-)! Also a little more expensive than the Marauder, last time I checked. Another very good option is buying used. The various forums also have their classifieds, I've bought most of my airguns that way over the years. It opens up your available pool of guns considerably. Later.
Dave