speaking as a person who has not lost a parent, i can't really say much about this, but i think that losing a spouse/SO is DIFFERENT; i wont speculate at which is harder.
there is no scale of suffering, and attempting to create one is hurtful to people. people who lose parents and people who lose spouses/SOs have lost SOMEONE of importance. people who lose a parent don't hurt more or less than those who lose another close person, like a sibling, a best friend, or a spouse/SO.
the difference i would think with parents is that most of the time they are preparing their kids for life as adults and, inevitably, without them.
it's an "understood" matter of life that your parents will get old and die.
it's understood that your spouse/SO will die, since we are all mortal, but in its own way it's not expected.
with your spouse, it's all "tomorrow, next year, the future." having children, getting old together. none of that stuff is expected with a parent. with your SO you create something to look forward to, a life. children that you want and pick names for become real people in your mind.
if a spouse/SO dies before that stuff is accomplished, you not only feel the death of the spouse/SO, but the loss of the FUTURE as you knew it, you feel the "deaths" of the kids who will never be born.
atleast that's how it is for me.
i never knew my father, so in a way he died a long time ago.
if my mom dies before me, i will be sad, but at the same time, i think i'm a bit more prepared for something like that. if you're a kid who experienced alot of losses/deaths growing up, you begin to deal with "what will i do if my parent dies, instead of someone else?"
matthew dying on what was essentially the first day of the rest of our LIFE... that is something that i don't know if i will ever come to terms with.
parents getting older and dying is the "natural law" for lack of a better term. and even though you know a spouse/SO will eventually also die, i think it's a really different experience.