Buy a Garmin. Yes, they aren't cheap, but they're worth it. Most computers can do speed and cadence at the same time (if they can do cadence).
Now for the details. Wired computers are great for speed, less ideal for cadence because of the long wire run to the cadence sensor. Its doable but time consuming to get a clean install. If you don't mind the wires, go this route.
Wireless (non-digital) computers are expensive and frankly don't work all that well. The wireless is subject to interference from bike lights, so you lose functionality as soon as you add a light.
Digital computers, such as Garmin, typically rely on ANT+ format (or Bluetooth 4.0) now. These do everything, and are reliable but not cheap.
My history:
Purchased a cheap wired computer with my first bike.
When I bought a new bike I got a wireless computer. Used it for about a year before getting tired of interference and drop-out issues.
Gave in and bought a Garmin. I should've just bought the Garmin in the first place.
Last edited by gsa103; 01-21-14 at 03:14 PM.