I don't know exactly what the differences are between the two apart from the fact that they don't look the same (I'm a total newbie coming into this). I didn't like the LSS and LSS/XL because of the very low profile fore-end on them. Wasn't sure if the fore-end would flex when using a bi-pod. I was looking at the MDT TAC 21 also but I saw some mixed reviews on them due to the fact that the scope is mounted on the rail/chassis instead of the receiver. I also looked at other stocks but with those stocks you had to get bottom metal etc (my SPS came as a DM originally), the HS3 covered all those things at once, so I went for the HS3. I can say 1 thing for sure....I love my HS3!!!