Another difference is the rank at which you can increase beams and disruptor weapon power with skills. You can buff beams at Lt. Cmdr. rank but you have to wait until Cmdr. to put points into disruptors.
That really isn't correct. The Beams skill applies to both Phaser and Disrupter beams (as well as the other flavors of beam weapons). Likewise, the Cannons skill improves both Phaser and Disruptor cannon weapons. Starting at Lt Commander, you can specialize in more specific weapon types.