Wired bicycle computers are very accurate if you take the time to set them up correctly. Roll out the distance-- don't use the charts in the manual. Put a bit of pressure on the tire to simulate a loaded bike.
Bike computers are more accurate than a GPS, actually. The bike computer will record your actual distance-- including the side-to-side wobbles. The side-to-side motion is too fine-grained for commercial GPS systems to measure. So, an accurate bike computer will have about a 1% difference from a GPS, I find.

The cadence function is definitely worth getting.