I've swatched a comparison of OPI's Stranger Tides and Sinful Colors Seaweed, which look like dupes to me. I couldn't see any difference in the photos or IRL. There was a difference, however, in the formula. Seaweed went on smoothly and it was opaque in two coats. Stranger Tides was a bit watery and took three coats to achieve bottle color. I'd say this choice comes down to personal preference: if you're an OPI girl and you love the wider brush, go with Stranger Tides; if you're not, enjoy the $6.50 savings and go for Seaweed.
The color, by the way, is a soft, muted, perhaps slightly vintage or even a little dingy, sage-green. It's pretty, and even though it's green (which would remove it from the business-casual section of most closets), it's a pastel, and therefore a stealth-green, which you could totally camouflage with an outfit involving a sweater set and pearls.