There's not much of a polyamorous community where I live, and those seem to be all single, twenty-somethings, so despite my initial prejudices (picturing cheesy 70's-style "wife-swappers") I delved into my local swingers' community as a way to connect with others my age in committed relationships, but open to non-monogamy.
Most of "the lifestyle" folks I've met preferred some level of friendship with their lovers, some even had morphed into very close relationships with other couples that verged on, if not met, the qualifications for polyamorous relationships.
The difference between poly and swingers, to me, seems to be:
1) swingers are a "couplecentric" society. The swingers I know strictly have sex with others as a couple. It is something hubby and wife do "together" to "enhance" their marriage. The idea that you can fall in romantic love with more than one person at a time is somewhat uncomfortable to them.
2) swinging is geared toward heterosexuals. Though it's accepted for women to be bisexual (emphasis on the sexual), most of the swingers I've spoken to seem surprised at the idea that a woman who isn't a lesbian might actually fall in love with other woman. Bisexuality in men is pretty much frowned upon or kept on the down-low.
Though I live in a hugely gay/lesbian/transgender-friendly city, I haven't met any gay/lesbian/trans couples within the swinging community. I know plenty of gay men who are non-monogamous in one way or another, but they do their own thing amongst themselves.
I've been dating a guy for the last month or so I met on a swingers' dating site. He's divorced/single, hetero. His reasons for meeting people within "the lifestyle" are that he likes to have sex with others watching, he doesn't do well with traditional "dating" as he refuses to engage in any relationship where monogamy is eventually expected, and because he travels for several months a year so he feels he can't commit to a traditional relationship. That being said, he seems to very much be enjoying our one-on-one dates together, and he's as attentive as any man in the first stages of dating a woman.
I think of swinging as a way to engage in non-monogamy while still adhering to many of the norms of heterosexual/monogamous society. Most of the swingers I've met were nice, easygoing people, and the couples are very much in love with each other. Polyamory seems to have a more political, feminist, queer-friendly bent, which speaks to me more than swinging. But, whatever. It's all good.
Early 40's female, bisexual.