KravtZGo to a boot fitter. You don't know till you try them on.
Personally I think the SPK is a horrible boot. Soft, way to much flex, not aggressive enough. I wouldn't touch it with a 10 ft pole. But thats just me.
Go to a boot fitter
that's entirely subjective though, without knowing the age, height, weight, and skill level of the skier. to my knowledge, the SPK is (was) made most recently in four different flex grades; 75 - 85 - 90 - 100. While flex rating is not universal across all boot brands, you can get a pretty good idea of the stiffness of the boot using these.
I have a medium width food, with medium-high instep height, and skied in the old black & white SPK. One problem I encountered was the bottom buckle, when buckled tightly, would pinch the top of my foot fairly hard, but not necessarily bind the outside of my foot to the point of discomfort. the flex was firm enough to drive the boot on steeps, but soft enough to have some forgiveness in the park (I was 6'1", 170 at the time).
OP, if you are on the smaller side both in terms of height & weight, you'll likely require something with a lower flex rating. If you are heavier (like I am now), you may want something stiffer. It depends entirely on what you intend to do with the boot.