If you simply hover your curser over an engram before you decrypt it, you'll see information about the item. For instance, a Legendary Engram has the options of Rare, Legendary or Exotic Gear. It's also a possibility of a random class (Hunter, Titan, Warlock). The weighting and scale of each engram is only known to Bungie. By scaling up your level at the Cryptarch, you increase your chances of getting "better" gear that you need. Don't get tied up if you don't get the exact level you want, just try to keep getting more engrams redeemed and build up your level and you'll reap the amazing rewards.