The orb is certainly worthy of respect, if only for its ability to multiturn shutdown an enemy at the higher levels, but I think that both the Staff and the Wand are competitive choices as well, despite the Wand doubling up Dex and Int on the Reflex defense.
The reason I think this is so is because all of the bonuses they provide are not factored to scale - that is, they can scale against variables that don't. The enemy's save against "save ends" effects are obviously of this nature, but the Wand's attack bonuses also are stacked on top of every other scaling attack bonuses, so its end effect is that it gives you a scaling advantage against an attack bonus/defense bonus relationship that's otherwise relatively stable.
What this essentially means in relation to the Orb is that the Wand allows you to nail an effect more surely, whereas an Orb allows you to keep that effect going more surely.
Assuming that we keep the relationships more or less the same, casting Sleep using a wand with a +6 Dex bonus improves the initial to-hit by 30%. If you're only hitting a Solo or Elite 40% of the time, you now will hit 70%, with an almost assured hit on a reroll.
If the enemy saves 80% of the time, then you chances to knock him unconscious for 1 round is about 14% on the first round, barring other influences; however, you retain an 70% chance to slow him.
Conversely with a +6 Wis debuffing the 80% to 50%, but not improving the to-hit, you have a greater chance (20%) to get him unconscious, but a lesser chance to get him slowed (only 40%)
As you advance in levels, you get the chance to recast Sleep on a round-to-round basis until you hit and then apply the Orb effect, but until then, you're basically doing nothing. It's like Orcus is status-locking you without even trying.
The Wand's effect is comparable to the Orb's in terms of Sleep, but MUCH better in that you can use it to much better effect on things that are so powerful than they don't even have continuing effects. What I'm talking about here is obviously something like Thunderclap. If I had to choose an encounter power I would be able to spam as an Archmage over and over again, it would have to be that. Orb doesn't help me to nail Thunderclap. Wand will. Once I hit with a Thunderclap, the next one is at Combat Advantage and gets +2 to hit. Even better, I status lock the Solo in a very solid way for 2 rounds.
If you coordinate stun-locks party-wide, you can status lock a Solo pretty much for most of the encounter.
This is much better than trying to hit the Solo with Sleep over and over trying to get it unconscious.
Of course, there are other ways to ensure getting your spell to stick, and there are also other ways to debuff an enemy's saves (Spell Focus for the latter, BattleMage for the former), but in general, I think the Wand's to hit bonus is a more powerful effect than the Orb's save debuff, which is why it got mapped to a redundant Defense stat and doesn't have secondary powers.