MIDI controllers come in a variety of sizes, ranging from 88 keys to a compact 25 keys. If you are a piano player, or plan to use your MIDI controller to compose music with notation software, a controller with a higher key-count may be more suited to your needs. If you are using your MIDI controller to input single-line melodies into a DAW like Ableton Live, or are looking for a portable option, a smaller key-count may be more appropriate.

MIDI controllers come with different types of keys, including weighted, semi-weighted, and unweighted. Users accustomed to the feeling of a real piano may prefer a MIDI controller with weighted keys, whereas those unfamiliar with playing piano may have no preference. It is worth noting that key type is one of the biggest factors in determining price, with fully-weighted keys being the most expensive. 

Consider how you want to connect your MIDI controller to your machine. The majority of MIDI controllers manufactured today pass MIDI signals through USB, so they can be connected to most modern computers with ease. However, some MIDI controllers may only send MIDI through a 5-Pin MIDI DIN cable; if you wish to use a controller that only contains 5-Pin MIDI outputs, you may need to purchase a separate MIDI interface or an audio interface with a MIDI input. 

Some MIDI controllers come with drum pads and additional control features like knobs, sliders, and buttons. These may not be useful for composing music in notation software like Finale or Sibelius, but can be crucial for triggering samples or writing automation in a DAW like Ableton Live.