When you're songwriting, you want nothing to get in the way of your creativity, and you want as fast a workflow as humanly possible—so for those reasons, you're better off starting the songwriting process with MIDI rather than recording audio (if not you're not a keyboard player, even a simple MIDI guitar controller like the Jamstik+ or You Rock guitar will do the job). Here are the two main advantages.
Transposition. You can transpose MIDI instruments quickly, while retaining sound quality. When you're looking for the right key for your voice, you can find it in seconds.
Tempo changes. There's a tendency when writing to play a bit more slowly because you're feeling your way around the chord progressions, lyrics, etc. Once you've established the song's framework, then you can experiment with different tempos until you find one that feels right.
To get started, my tool of choice is a multitimbral synth like IK Multimedia's SampleTank, with a preset that contains the kind of instruments needed for songwriting. Then it's possible to lay down multiple tracks quickly to create the song's overall shape, which makes choosing the key and tempo just that much easier.