If you're looking for a digital guitar tuner, the first thing you need to do is consider what you will be using it for. If you are going to need it in a noisy environment, then you'll definitely need some form of plugin, so that you can plug your guitar in directly without having to rely on the built in microphone. In general, these kinds of tuners are recommended.
You can also get a guitar tuner that clips onto the head of your guitar, and gauges the tuning by the vibrations that travel through your guitar's body. These ones work fairly well in noisy environments, but would still not be recommended in a performance situation.
You can also get pedal based digital guitar tuners, allowing you to remain plugged in the entire time, with the ability to see your tuning easily from a standing up position. These tuners are a bit more expensive, but ideal for the performing musician.
The best thing is to do your research before selecting a tuner. Make sure you read the reviews on the one you're interested in, and you'll be sure to select a digital guitar tuner that suits your needs.
