DR works like a “whitelist”—it tells you what damage it allows through, and it blocks everything else. So DR \$x\$/magic reduces damage by \$x\$, but not if the attack is made with a magic weapon. And DR \$x\$/– is an empty whitelist—it doesn’t allow anything through, it just reduces attack damage by \$x\$ unless the attack has extra-special ability to overcome DR (as with a paladin’s smite evil).
As a result, we can draw some conclusions about the potency of each form of DR:
DR \$x\$/magic is basically worthless past about 5th level. Every serious threat that is trying to hit you with physical attacks has a magic weapon that ignores that DR (creatures who have DR \$x\$/magic themselves count their natural attacks, like claws, as magical for overcoming DR, so even most beasts ignore it). You (or a monster) could have DR 1,000,000/magic and it wouldn’t matter at all after the lowest levels in the game.
On the other hand, DR \$x\$/– is not ignored by any physical attacks at all. All weapons will have their damage reduced when attacking DR \$x\$/–. In principle, that makes DR \$x\$/– very good, because it is perfectly reliable—any time you are subject to a physical attack, it applies.
However, in practice, DR \$x\$/– is also very low-value for players. Quite simply, you can never find high enough \$x\$ values to make a large difference, and the cost of getting what you can is far too high for the small difference they make. Pathfinder charges a lot (in terms of feats, class levels, gold, etc.) for even tiny amounts of DR \$x\$/–.
Ultimately, DR is much more the province of monsters than it is players. Monsters come with DR, often DR \$x\$/material or DR \$x\$/alignment, that is large enough to matter, without necessarily having to “pay for it” the way players do. Some even get decent chunks of DR \$x\$/–.
Note, however, weapon enhancement bonuses can overcome material-based and alignment-based DR. Monsters having that DR still matters, unlike the DR \$x\$/magic, though, as the existence of those DR values strongly encourages players to get those enhancement bonuses instead of special weapon abilities (unlike D&D 3.5e, where enhancement bonuses are basically a waste of gold and no one should ever get more than the minimum +1). So such monsters are “protected” from more powerful weapons by forcing players to get the (otherwise-weaker) straight enhancement bonuses.
At the highest levels of the game, though, DR \$x\$/– is the only form of DR that will actually come into play. It’s still usually too small to make a big difference, but some monsters are exceptional in that regard.
Finally, keep in mind that DR, even DR \$x\$/–, only ever applies to physical attacks. Magic energy attacks and the like don’t interact with DR at all. Resistance and immunity are similar concepts that apply to magic (and not physical attacks). However, resistance is a blacklist system, as opposed to DR’s whitelist system (that is, resistance tells you what it blocks, and it doesn’t block anything else; DR tells you what it doesn’t block, and it blocks everything else).