It depends on what you are willing to pay and what you want to do with the info. I bought a delta from PB that was wireless and showed 70mph when I looked at it by the computer to record the info! I picked up a couple of Planet Bike 5 for $5 each and use them for my family members who don't care much about data but want to know how fast and how far they went. For my bike, I choose to buy a Cateye wireless with the cadence bult into the unit that also picks up the wheel. What I like is that it is not affected by computer being near by and it measure off the rear wheel instead of the front. I can use it on a trainer. It was reletively expensive and the display is marginal for my poor eyes. Fortunately my coworkers gave me a gift certificate that I used for the purchase when it was on sale.
Mounting options is another consideration. The Cat Eye allows for multipple orientations of mounting. Mine is on the stem. Usually lower priced ones mount on the handlebars. Check out the reviews on http://www.roadbikereview.com/cat/tr...S_1624crx.aspx