The Mk XV vertical special hits a good bit harder than the Mk II, in addition to being more readily available.
I wouldn’t say the Mk XV is a noob trap. The moveset is simplified, but it kills things very well. The lights are marginally less powerful than the Mk II combo (with the Vanguard → Relentless switch), but if you have buffs from your talents or blessings active, I don’t think it’s a very noticeable difference as far as horde clear goes.
Mk II is more interesting to use (from a combo perspective), but I don’t think it performs significantly better.
My only fault with it is the speed; it gives the feeling of swinging a concrete block around. I enjoy both variants, but any time I look at the XV, I’m asking myself “Do I really want the slow weapon?”. I mostly play with Inexorable Judgement, but when I played witth the Mk XV and Blazing Piety, I really felt the sluggishness.