the horizontal pull-up is a great way to hit back and arm muscles for people who struggle with conventional pull-ups.
the differences between these two choices are reduction of weight, and also changing the angle of your torso. when you change the angle from vertical to horizontal, if you pronate your hand and go wide, you are moving the high demand from lats muscles(lower back) to rhomboid muscle, teres major, and rear delts(and other muscles which are located in your upper back), and when you supinate your hand and get a narrow-grip(shoulder width or a bit closer), you target the biceps with much stronger contraction, but less stretching at the end, and also the activation of front delts are going to be higher.