To clarify, The Echo is the venue for some of the lesser-know indie bands of Los Angeles. It's the Troubadour to Spaceland's Whisky. The Echoplex is the fun dance party scene, that hosts such widely-known nights like Bootie LA, Dub Club, and RAID. In general they tend to mix it up, like hosting the White Stripes at the Echoplex and the seminal punk rawk dance party Part Time Punks at the Echo.
They've got the essential indie rock attitudes balanced with a few carefree indie rock dance parties, and they tend to book a more subdued sound (hello, shoegaze, shoegaze pop, ambient, and ambi-drone). Fortunately, if your night at The Echo is a miss, you can always spice things up by heading down to the nearby Echoplex. The one stand-out thing about the Echoplex is that its dance parties have been some of the longest-running dance parties in Los Angeles, and they've set the standard for Booties worldwide. Because they've been so long-running, you can be sure that there's something good going on down there. Even if you just go to stand around ironically, checking out all the fashions of the kids these days, at least you can practice your hipster-cool-ennui expression to some really good music.