So far, fourth edition is all about teams. Yes, there will be heroic solo monsters and epic monsters, but they will be tough. Remember how that pit fiend had a +2 to saving throws? That's what they are talking about, and he's only half of a solo monster! Plus, he had a bunch of baddies with him.
The difference between save or die and sleep is how quickly it takes effect. The first round you know you're falling asleep. If you fail that save, you fall asleep. Next round you make another save until you wake up. This continues until you make the save. You have a 55% chance to succeed. Everything has a risk factor, but 4e has much, much less instant fails. I'm not sure why you like save or dies, honestly.
Lastly, most encounters, if properly designed, will not be alone. They will contain many, many monsters. Thus, if you sleep one or two: Great work! Either deal with them now, or move onto more pressing matters. But I guarantee you -- a properly made encounter will not let one wizard slay the entire band. Now, if you use sleep on a big bad, then more bonus for using tactics.