I get regular questions as to whether to use a play along track or a metronome.
I guess it depends somewhat on the purpose: both are equally capable of helping you with your time. At the same time however when it comes to using it for the purpose of improvisation, I generally prefer the metronome. While having the whole band behind you, for instance using Band-in-a-Box or play along CD’s may seem great, I generally find that they are a limitation to my imagination. I can’t help but feeling that when it comes to improvisation practice: less is more. More than once it seems like the play along tracks direct you in a certain direction. So, I would generally prefer a metronome.
At the same time however, I realize that it may well not be that easy to hear the harmonic structure of a song and improvise on that especially when you are starting out. I guess that is where I’d go for using a midi file or band in a box file with just the bass or bass and drums.