I've got a BD Express now, and it shoots pretty well, less than 2" with a couple of different loads. I've also have a couple of Knight Elites and Extremes. All of my Knights have shot less than an inch with preferred loads. I haven't been able to get accuracy that good out of the BD Express. The diifference between the Extreme and Elite is the barrel for the most part. Elites have a lighter contour barrel with no sights, while the Extreme has a regular contour barrel with sights.
As for the Disc's, I like them because they are easier to handle than the primers by themselves when you are doing a fast reload. When shooting for practice, it doesn't make a difference to me.
You would be coming out way ahead money-wise if you can get an even trade for your BD. Even though the Elite was dropped a couple of years ago, they are still selling on the net for $350-$450 NIB. I bought my BD for $180 shipped, NIB. The BD carrys a lot better than the Knight, as it's lighter and more compact, but I don't worry about that myself.