It's all up to personal preference. Skillspeed inherently has increasing returns(though very little) so there are some arguments to be made to take it over determination. But in the end it all comes down to what you prefer to play with as long as you aren't gimping yourself with too much accuracy or taking only skillspeed over crit. Having certain skillspeed values in certain fights does allow you to pull off some mildly interesting things. With a skillspeed value of about 640 you can reliably hit the first add in the second phase of thordan with 10 gcds and keep GL3(if you end with a snap punch) until the next two adds spawn. Of course there are some other ways to pull this off by throwing in fracture or something of the like.