The text will remain sharp and clean at any point size, resolution and scales linearly. Since TextMesh Pro uses a different text rendering technique, only one font asset is required per font type to render at any size. Furthermore by using different materials (presets) you can change the look of this font to dynamically add outline, shadows, bevel, glow, etcā¦
I have created several videos which explain this varying degrees but this more recent one goes over it quickly but will also give you a glimpse of additional functionality in the tool over the UI Text in Unity 4.6.
Only one single font asset is used for all point sizes and resolutions. Scaling must be uniform (ie. X, Y, Z) most people forget about Z which would cause the text not to render correctly.
In terms of performance, the TextMesh Pro UGUI component uses the CanvasRenderer and as such inherits the same overhead as UI Text or anything rendered using CanvasRenderer. The normal TextMesh Pro component which uses the Mesh Renderer should still be used for text in a scene as it doesnāt inherit this additional overhead and thus offers better performance.
BTW: UI Text or TextMesh Pro will have similar performance, the performance overhead comes from the Canvas system and Unity having to do extra processing to try to batch all these elements into fewer draw calls. As Unity improves that part of Unity 4.6, performance will improve for all components using the Canvas system.
In terms of Text Control, TextMesh Pro offers substantially more control over your text than UI Text. The previous video should give you a good idea but just in case, TextMesh Pro offers control over character, line and paragraph spacing, kerning, numerous additional rich text tags to control text alignment within the body of the text or indentation, new page overflow mode, support for superscript, subscript, smallcaps, and more. Here is an image of a few of the newer tags added.
Example showing some of the tag that provide control over text alignment within the body of the text.