I think Spike didn't care about dying. That's why he went for the toughest targets, slayers. It was only later on, once he'd met Buffy, that he made the decision to live. 😀
I think Angelus didn't give a shit about dying. (Am I allowed to curse in this forum?) He openly mocked The Master, knowing he could get killed for it, and even face down on the ground, after getting his ass kicked, he still defied The Master and encouraged Darla (The Master's favorite child) to leave with him. He also rejected The Beast's offer while, again, expecting to die.
Angelus had a set of principles he lived by: he rejected love entirely, liked to live large, always had to be the biggest Big Bad in the room and worked for no one. And if defending this got him killed, so be it. Even during season 2 with Buffy, I think, no matter how arrogant he was, he knew he was playing with fire. He knew that Buffy probably wouldn't kill him, but definitively could. Yet he didn't back down from a challenge. Frankly, his indifference to death makes him particularly dangerous. The thing Angelus seemed to care about the most was his pride. Besides that, there was nothing he feared losing, unlike Spike, who loved Dru and his comfortable existence. Angelus was willing to suck the world into hell even though it meant losing a lot of things he actually liked on Earth. Spike, on the other hand, thought that was crazy even for vampire standards. His indifference it's probably one of the reasons he was so patient while inside the cage. He didn't seem to really fear getting his soul back.
I think Angelus lived in the now. That doesn't mean he wasn't patient and willing to wait for the right time, but he was all about making the best of every moment. I kind of admire, in a way, how joyful he always is. He lives moment by moment doing things he likes (Darla, Dru, killing, maiming, turning friends against each other, watching ballet). He doesn't fear death because that's a concern for the future. Like I said, even in a cage, he made the best with what he had, knowing he would get his chance for freedom, but for now, he was just enjoying ruining everyone's already fragile relationships.
So, to conclude (why can't I ever be brief and to the point?!) Angelus is the bigger threat for all the reasons above mentioned and, like everyone else said, because Spike would just kill you and Angelus would make you beg for death.