It's all about the pivot joint
Whether onstage or in a studio, few things compare to the horror of watching your boom-mounted microphone slowly slipping down away from your mouth, instrument, or speaker cabinet; the Posi-Lok design puts an end to that. This results from using a locking, toothed mechanism at the pivoting joint of the boom, as opposed to the friction-type clutch used with an ordinary boom. I just wish that On-Stage stands would incorporate this type of pivot into their regular booms, and eliminate the friction clutches entirely; I'd gladly replace all my friction-clutch booms in a minute.
For versatility's sake, I would recommend getting at least one each of the Telescoping Mini Boom (MSA9505) and the Side Mount Boom (MSA9508). Note that the two halves of the locking pivot mechanism are interchangeable: that is, for example, you could take the telescoping boom arm and attach it to the side mount clamp. Another handy arrangement is to have the telescoping boom attached to the top of your stand, and extending out for vocals, while the side-mount stand is clamped on partway down the stand, in position for your acoustic guitar. Put a little counterweight at the bottom of the stand and you're ready to go.