I have the 15 and its not too long when you through a t-stock on there. It doesn't get in the way and, like others have said here, allows you to use a whole 10 round tube and still have a few balls in the tube in case you need them. I play with a group of guys with electros and you need that extra 2-3 shots and its nice to have them ready to go should one of those opportunistic fellas try to rush you when you are reloading.
But, the beauty of the phantom is its ease of configuration. Just buy both tubes for a few dollars more and you can screw in which ever feed you like or need for the task at hand.