[Scribus] RGB vs CMYK

[Terence Chan]

Hi,

I'd like some advice as to which of the following options
will be more likely to give better results when printing
Scribus document at a commercial printer: importing RGB
images into Scribus and using an appropriate CMYK printer
profile when saving to PDF or converting RGB images into CMYK
first using ImageMagick (convert -colorspace CMYK) before
importing them into Scribus and again exporting to PDF?

I've experimented with both and the soft-proofed images
appear quite different on screen as well as in PDF, but
maybe this is just because the ImageMagick conversion isn't
very good and I should be using another image processing
software to do the RGB to CMYK conversion (or leaving it
to the printer's own RIP)?

Terence