I'd be wary about using phone apps, because of the potential for draining the battery and not having a phone when you need it. Of course, I do not possess a smartphone, so take it with a grain of salt.

If you don't go with the phone, you have two options:
1) Wired. Sensor mounted to the fork (zip ties) is directly connected to the computer. Advantages: Cheaper, less prone to interference (from power lines, other bike computers, sensor things at intersections...). Disadvantages: Can be a pain to mount, especially if you get a model with a cadence sensor. Often don't have the same features that wireless models do.

2) Wireless. Advantages: Mounting everything is easier (zip tie sensor to fork, and attach the bracket to the bars or stem. Voila! No wires to fuss with, and your bike looks cleaner. Disadvantages: Prone to interference, more expensive.

There are also GPS units. Garmin makes several. I don't know about those.