The pistol takes time to master. Most dive in too early but they soon realize that they need to lay down a good foundation before they attempt the pistol.
I recommend mastering the airborne lunge first then progressing to the pistol. Here is a quick video on how to do the airborne lunge.
I hope that video helped with a good progression for the pistol. The airborne lunge will smoke you.
The bodyweight pistol requires much more mobility than the weighted pistol. You need to utilize almost all available ankle dorsiflexion with this variation. You weight yourself in the middle of the foot and not just your heel.You will also have a torso position that is more forward and "over" your legs. There is generally more lumbar and thoracic flexion.
The weighted pistol is easier for a few reasons. One, you have a counter-balance. This allows you to sit back earlier and shift the weight towards your heel. Two, you can get away with less than adequate ankle mobility because of this counter balance. The knee does not have to glide forward as much because you can sit back. The last key difference is the torso position. The weighted pistols allows the spine to be taller throughout the movement as well.
Here is a quick video on the differences