Both good, and the true Mk4's are tanks.
The difference between the true Mk4's (the fixed power scopes), is they more than double the elevation adjustment over the variable so you can shoot WAY farther, and are much more rugged (less to go wrong as well). The fixed power scopes are true military optics and designed that way from the get go, vs the variables which are 'tacticalized' VX-III's. That's why the most expensive Mk4's are the simplest ones, the fixed power 40mm's, the 16X retails just under $2000 plus tax typically, but you can pick up a 6.5-20x50mm for a lot less.