I use a Cateye Strada Cadence (~ $48.00) wired computer on the road bike and a Sigma BC 509 ($23.00) that on the hybrid. The Sigma has MPH, distance, ride time, odometer and clock. Pretty much what I need and use most often. For me, the reason for the cadence on the road bike is so that I can use the computer on the trainer. On the Strada, both sensors are mounted on the chain stays; the speed sensor on the inside of the stay and the cadence sensor on the outside.

I see cadence as a tachometer on an automobile. Some have them and some don't. You really don't need to know how fast the engine is turning when you are doing 55 mph in 5th gear. At 55 mph, the engine will always turn the same rpm in 5th gear (+/- some rpm's). Since the two go hand in hand, maintaining a given speed requires that you use a certain cadence in a selected gear. As CraigB said, you can calculate your cadence relatively easy. Don't get me wrong, it's nice if you have it but to get cadence on a computer you will pay for it.

Average speed is also a nice feature to have but is very misleading at times. If you start riding and hit your normal riding speed (let's say 15 mph) right off the bat and do the entire ride without stopping, your speed will fluctuate +/- some mph and average speed will be fairly accurate. But if you slow your speed down for any reason or have rides where you have several stops, average speed goes out the window if you are using it to track your progress. When I ride, I normally ride between 17 and 20 mph on the road bike. I occasionally slow down to drink or get a gummy chew from my jersey pocket or when there is a headwind or the bike path is very crowded. At the end of my ride, the computer shows an average speed from 14 mph to 15 mph just because of the times that I slowed down for whatever reason. And again, its nice to have it if it comes on the model that you select but I don't really think it is necessary.

Other opinions may differ......