Henke, I agree with Krum: there's no need to get unfriendly. He asked a serious question, concerning two features that really are similar enough to get confused with.
Please, I want this forum to be a friendly place where anyone is welcome. I'd appreciate if you'd be a little more careful with your tone -- I know you're a competent Starling user and value your help, as long as it's positive and constructive. Thanks!!
Krum, you're on the right track. A RenderTexture really is exactly what the name suggests: a texture just like one you've loaded from a file. The only difference is that you've created it on the fly. So it will always be exactly one draw call -- however, it will also take up a significant amout of memory (depending on its size).
To understand a QuadBatch, think of one big mesh of triangles/quads that are sent to the GPU in one call. When you use a flattened Sprite, you *are* using a QuadBatch, because that's exactly what's used behind the scenes by a flattened sprite.
The advantage over a flattened sprite: you don't carry the weight of the display list with you, e.g. adding something to a QuadBatch won't dispatch an "ADDED" event.
The advantage over a RenderTexture: it doesn't require much additional memory (just the vertex data, which is not much compared to texture data).
The disadvantage over a flattened Sprite: it's a little more difficult to use.
So, a QuadBatch is the most efficient way to create a tilemap. Good luck!