Is it better to convert a photo in Illustrator or in Photoshop?

I have run some tests in both Illustrator and Photoshop to examine whether Illustrator’s Convert To Grayscale capability is press worthy. While Illustrator does convert the image to grayscale properly and running colour separations demonstrated that only the black colour is used (while Cyan, Magenta and Yellow are not used at all), the quality of conversion compared to Photoshop is not good. The problems arise when using the Convert to Grayscale function on a file that contains raster elements.

If someone wants to convert an Illustrator document which contains raster images to grayscale, I thoroughly reccomend NOT to do it. This is for two reasons:

1. Illustrator cannot convert images that are linked. So one has to embed the image into Illustrator and should there be a problem with it when going on press, it will be difficult to troubleshoot the problems.
2. Illustrator conversion to grayscale is not as good as Photoshop as I said. The image looses detail, which is retained in Photoshop. So if the first point doesn’t turn out to be a problem, this one certainly is.

The way I would go about it, if I had the original raster image, would be to convert it in Photoshop first. The if one wants to add some vector elements in Illustrator he should import the image in Illustrator, leave it linked and not embed it and then give Illustrator file and image to the printer (and fonts if applicable). If one wants to try, he can embed the image in Illustrator that he previously converted to grayscale with Photoshop, but this last method is a bit risky.

Having said all of the above, I don’t see any problem in using the Convert to Grayscale function in Illustrator if the file you are working with has only vector elements.