I have a bad knee, so I'm fairly familiar with all the straps/bands/braces that are offered. I'd go with the strap, as opposed to the "sleeve", simply because I always found that the sleeve (i.e., basically a sort of brace) actually aggravated my knee.
If you want to go really cheap, you can do what I do when I'm out and don't have my strap with me - get a pair of knee high stockings, cut off the foot on one of them, and roll it up to just below the knee. It exerts just enough pressure to ease my tendonitis.
JEAN
2011 Specialized Ruby Elite - carbon fiber go-fast bike
DiamondBack Expert - steel road bike
Klein Pinnacle - classic no-suspension aluminum MTB