Perhaps TTF is better if good hinting is included in the font (not always the case, read the state of webfont quality by Ross Mills on Typographica) BUT as 99% of fonts are designed in cubic Bézier (PostScript) curves, conversions1 to quadratic Bézier (TrueType) destroy partially the curves, so the design. Even if you try to correct the new outlines in “TrueType” with the cubic Bézier original curves on the back, some details will be lost. Its really depending of the nature of the design. Few years back, when Linotype launched OpenType pro fonts, they decided to go for OpenType TTF flavor (quadratic Bézier — TrueType). I was fully against in the case of the conversion of my Sabon Next for its Pro version: This image compare on the back (yellow) a detail of the final serif of the glyph a in cubic Bézier versus on top the TTF version…
Never use Sabon Next on its TTF version for poster sizes… outlines are not at the quality of the original. Its why the Pro version in TTF isn’t available directly through the Linotype webstore (hopefully).
Perhaps in the future if a good TrueType “quadratic” editor (+ easy hinting) — easy to use as it is with PostScript cubic Bézier — is launched, typeface designers will design directly in this outline format…
1 (Beat Stamm comments) Technically, TrueType curves are quadratic Bézier curves, while PostScript curves are cubic Bézier curves (see 4 Applications), both of course invented by Pierre Bézier for designing Renault cars. It is theoretically impossible to represent an arbitrary single cubic Bézier curve by a single quadratic one, but one can get as close an approximation as one desires by converting a single cubic Bézier curve to more than one quadratic ones. The trade-off is more off-curve points (think of it this way: a single cubic has 2 off-curve points, and so do two quadratics in a row). Hence the achievable quality really depends on the quality of the conversion, on how many [off-curve] points are tolerable (=> file size!), and on how useful are the editing tools for making final adjustments.
Updated on 28 april 2011, following exchange of emails with Beat Stamm.
More ressources about rasterization:
— Texts Rasterization Exposures.
— A Treatise on Font Rasterisation With an Emphasis on Free Software.