I cannot give any other answer to this question than BOTH.
Panzer Dragoon Saga is one of the finest RPG's of all time. It's only flaw is that it is a bit short (15-20 hrs.) But aside from that, the music is some of the best ever, the battle system is unique and always a blast, the story very compelling, and the visuals are definitely some of the Saturn's finest.
Radiant Silvergun is, arguably, Treasure's finest hour. A rockin shooter with six different weapons for your ship, plus the Radiant sword which can swat certain types of projectiles and build up a gauge that once filled will allow you to perform a very bitchin invincible attack and inflict gobs of damage on everything nearby. It also sports some very cool anime cinemas, a great orchestral soundtrack, and definitely pushes the Saturn to the limit with every special effect in the book, huge bosses, parallax scrolling, etc. Great replay value in finding all the bonus 'dogs' (hidden spots you shoot) and making chain combos by destroying the same color enemies in a row. Fucking top flight shooter.
I'd say if you have to choose between the two, you should just go by your genre preferences. If you dig shooters, by all means, Radiant. If you love RPG's, pick Panzer. These last generation Saturn masterpieces are what etched this console's place in history as the "32-bit Turbo Duo." If you think about it, it's a pretty apt description, right down to the fact that they both had add-on carts for additional RAM for certain games. Speaking for myself as a gamer who wants to experience the best of everything, I'd have to say they're both an absolute must. They don't command top $ on EBay for nothing.