Helldivers 2 is arrowhead's first foray into a game that's not top-down, which makes it more impressive. It also allows Helldivers 1 to remain unique and enjoyable on its own, because the two play quite differently, and I still really love the first one
1 is done, but it's had more time to receive updates, so it has plenty of things to unlock and use. Arrowhead basically took what they could of Helldivers and translated it to 3D as best as they could, sort of like Risk of Rain 1 to 2, so some knowledge and all stratagem inputs will be similar.
As long as you're cool with top down shooters I think it's still great