For me the key element of HN is discussion based on evidence and logic. By that definition (and most others), I consider bogleheads.org to be the best investment site out there.
The investor topics on HN likely won't be from an investor's perspective. Quora is quite formal, so you'd get more "investor to general public" posts than "investor to fellow investor" posts.
[+] [-] tempestn|13 years ago|reply
[+] [-] _delirium|13 years ago|reply
[+] [-] stevievee|13 years ago|reply
If you are a professional, I can't see why you would want to critically discuss investments in a public forum.
[+] [-] eddiehobbes|13 years ago|reply
both link to high quality articles about investing and the economy
[+] [-] ig1|13 years ago|reply
That said most investor-type topics are likely to be on-topic on Hacker News as well.
[+] [-] jfoster|13 years ago|reply
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[+] [-] loue|13 years ago|reply