Each of the sevenths has a very distinct sound. Okay, you should be used to the drill by now - play each chord and listen to the differences. Personally, I'm very enthralled by major seventh chords. They have an almost exotic flavor to them. Even when I'm writing a rock song I will toss a few into the mix to spice things up.
Regular sevenths evoke quite a different response, one of unrest, of unfinished business.
They always seem to be leading somewhere else. Now let's test this out and see how the seventh chord flows into the fourth of any given scale. One really does seem to complete the other. If we're in the key of D and we play a D7, we'd follow it with the IV, which in this case would be the G chord. If you can train yourself to recognize seventh chords, then you will almost subconsciously find yourself learning to pick up the fourth as well.