D2 has better corrosion resistance than 5160. It has better wear resistance as well. 5160 is much tougher and is used in the cutlery world for chopping, or large tough knives and even swordmaking. It is a spring steel by design. It sharpens easier than D2, and will corrode quicker depending on things like finish and coatings.
Both are excellent steels and , like all steels have their niches where they are very useful, and are very good at what they are good at.
I have the D2 Kabar Extreme and in my opinion if it broke from anything it would be prying type stuff first. The handle/grip area would be the next area to break after the tip. It's not weak or delicate at all and would only break if exposed to forces or uses not designed for. It is an excellend knife.
I also have several Ontarios in 5160 and like those as well. They have everything from light'ish .1875 inch stock up to .250 inch in their ranger line ( .3200 ish of S7 in their Ranger signature line). The Spec plus 43 is 8" long, 1875 inch stock which makes it a light, good feeling knife with strength probably not that much greater than the D2 Extreme. Not enough to cause me to chose one over the other. I'd go by feel, and projected uses more than any other attributes as both knives are competent, well built with the D2 Extreme probably having the edge for me in a self defense knife with the training I'm most familiar with. Just as a chopper, slicer either would do it with the Spec plus having the very slight edge in slicing stuff. I'd want a smaller knife than either for detailed work.
Good luck.
BTW, the 5160 really comes into it's own with the big Ranger 9 inch, .250 inch ( quarter inch thick) choppers. The Big , super thick .330? S7 signature knives in S7 are kind of indestructible feeling knives that have the ergonomics of a railroad spike.