I'm looking to take some film with me on an upcoming trip and am not sure which to buy. I don't plan on making physical prints of the photos, just having them developed and then transferred to a CD.
Colour negative film has a wider dynamic range than transparency film. Depending on the quality of the scanning at your chosen developers, you may be able to get more detail in the highlights and shadows using negative film, and you will also have greater latitude to correct over- or underexposed photos.
Slide film, on the other hand, (especially Velvia as ysap has indicated) generally has much more saturated colours, and depending on what you will be shooting, will produce images with more immediate visual appeal. Velvia is also a low ISO film which means finer grain and hence, effectively, higher resolution. Whether this will be relevant to you depends on the resolution of the final digital files.
There are low ISO negative film brands available, but they are not as common as slide film.