Tryst is a great alternative to Pure. (I do not recommend Pure, EVER.)
Tryst is beautiful and breathtaking. The outdoor candlelit patio and huge waterfall give this place an understated, upscale vibe. The music is good here, really mixed. The drinks are good. The crowd is usually really mixed as well.
I like Tryst because it is simple and classy, but not stuffy and snobby. There are no over-the-top frills to the place, just gorgeousness. The dancefloor is pretty big, and if it gets hot, you and your crew can take a time out on the patio, which is really refreshing.
The VIP tables are nice and not completely in the way, or out of the way. There is good people watching here, and a lot of bartenders and bar space. It gets crowded so get there early. Definitely dress to impress. (But why wouldn't you? you're in VEGAS!)