I expect with the likes of the microphone and battery inside the neck the in-ear buds will be lighter, and more battery life. If you read the owner reviews of a lot of Anker headphones its either a complaint that they fall out of the ear or they don't last long enough on battery, so one way to please the market is have the weight around the neck so you have more battery life and it should be lighter on the ears.
The slim replaces the IE20, this new Lite is more for replacing NB10.
I'm getting the Slim, on next discount code, I lost my IE20 (I didn't lose, I left in a meeting room and the person said they'd post them to me didn't). I see the Easter giveaway may be offering the Slim.