I don't use Reddit enough to justify paying for it, but I would certainly pay to support HN.
It's worth mentioning that moot has managed to keep 4chan running since 2005 without accepting any donations, while paying for what I imagine is a ridiculous amount of bandwidth and being shunned by most advertisers. Up until the addition of 4chan passes, he was paying a fairly large portion of the monthly server bills out of his own pocket. Moot hasn't said how many people have purchased passes, but I'm guessing it's an amount closer to "allows me to break even without dipping into my own pocket" and not "I can support 4chan entirely through user contributions!"
I think Reddit would have a much easier time supporting itself entirely through non-intrusive advertisement, which makes me much less sympathetic towards their desire to have users directly support the site.