If you want the ability to shoot conical bullets then without question the White Rifle is the choice to make. White rifles are built like tanks. You have to handle one to really understand why the owners of them are so anal about them. I personally own three of them. If I were asked to sell off my rifles I could agree except for my Whites. They shoot conical bullets better then you can hold them, and Sabotloader proved that with plastic and pistol bullets they can hold their own with any rifle.
There are parts for them. I believe White Rifle company might be trying to start back up. Also there are some previous owners that still have parts.
If all you want is a sabot shooter, then the Knight is a great choice. There will always be knight rifles around but the Whites are getting harder and harder to get your hands on.