There are several reasons I use a wired computer:
- Much much longer batter life
- No errors in readings due to wireless (you don't go under a powerline and suddenly it says your max speed was 80mph)
- No headlight interference issues

I just find wired far more reliable.

Theoretically you can also buy either a more expensive wireless computer that uses a more sophisticated communication system (so like $100-$150) that avoids simple lights interfering. Or you can buy a different light that's shielded against emitting the stuff that interferes with the radio waves.

However, my dad has a bike that he breaks down and puts in a bag for travelling, and I thought I would get him an expensive wireless speedometer and that should work. Bought one from Trek. Cost over $100, on his first trip the damn thing claimed his 5 mile ride was 60 miles. :-/

So I say - stick with a wired speedometer. They're just soooo much less hassle.