It doesn't really matter if the artwork is "at size".
If you are scaling artwork upon output, then a PDF is really the only viable output from a vector application such as Illustrator. You'll want the art to remain crisp. As vectors it will, as a tiff and pixels it probably will not.
PDF/X-1a does not support transparency. It will flatten all transparency. PDF/X-3 or greater will support transparency. I would generally recommend either PDF/X-1a or PDF/X-4. There's not a whole lot of call for anything in between.
Tiff doesn't typically support transparency either (Yes I know Adobe's tifs may appear to have transparency, but that's an Adobe trick - using underlying native data that is not inherent to the Tiff format). Colors should not be any different really. Except that a tiff will immediately flatten everything. If flattening causes color shifts, well that's due to flattening not the TIFF format specifically.
Tiffs can be smaller in file size, simply because it's one image rather than a whole lot of vector math data. If that's a concern. Generally it's not though. And if there's any apprehension about a vector PDF not being read correctly, a tiff certainly removes all that.
Ultimately there is no solid, universal, answer to this question. Each piece of artwork may or may not benefit from either format. In addition, any production house may be better suited to handle one format over another.
Now in terms of delivering artwork to a client.. I prefer Tiff when possible. Why? Because Tiffs can't be edited as easy as a vector PDF can be edited. Unless, of course, the client is paying for the vector editing ability.
