To expand further on darkarn's post: the Ember is the most powerful amp. And by "most powerful", I mean that technically—it has the most output power across the board. That means it can drive everything from planar magnetics to high-impedance inefficient dynamics and low-impedance efficient dynamics. The other amps are more limiting than the Ember and none have better specs than it. In short, that means that you should try to get the Ember if you can afford to.
The Horizon is the amp to get if you can't afford the Ember and want to drive only specifically high-impedance or inefficient headphones (AKGs, Senns, Beyers, et al). It's sort of comparable to Schiit's Valhalla in that sense (but is cheaper than the Valhalla).
The Starlight is the obvious budget amp and is spec'd out to drive most regular dynamic headphones reasonably well, but it won't drive ultra-high-impedance dynamics (i.e., don't use it to drive 600 Ohm headphones) or planar magnetics very well.
I can't really tell exactly what void the Sunrise fills, it's sort of in-between the Starlight and Horizon with both its specs & price but doesn't offer anything particularly unique. From my point of the view the main standouts in the G1217 line-up are the Starlight, Horizon, and Ember. The Ember is so "creme de la creme" yet affordable that most people should probably just go straight to it and skip the others.
Edited by Asr - 12/9/13 at 1:25am