I've got one 1300 Defender 8 shot, and a few 870's. I carry the 1300 though when in the bush, even over the 14" 870's. It's lighter than a 14" 870, has more capacity, and I can fire it quicker.
As for whomever mentioned unlocking on firing, YES it does, you're absolutely right, the 1300 was designed to do this. Whether or not this is positive to you is up to debate, I'm not sure I like it but the thing is fast. If I fire a 3" mag off a rest while not holding the forearm, it will go far enough rear to eject the empty shell all by itself.