I started with (color) negative film then switched to slide film for one specific reason: I wanted control over my pictures. With negatives, unless you develop and print yourself (which is a convoluted process for color film), when you drop your film at the local shop, you have almost no control over the look of your prints (exposure, contrast). And with negative film, it's hard to judge how good your picture is without a print (negative has crazy colors).

With slides, it's very simple: the process is the same for all slide film, the result is always the same, and the slide you see is 100% your picture, your work. If you under- or over-expose (on purpose or by mistake), it's your work, not that of the machine that made your prints. Plus, colors on slides, especially saturated films (eg. Velvia), looks worlds better (to me), but that's a matter of taste.

Also, when developing slide film, prints are extra, but I don't spend money for any print up front: I select those pictures I like and scan them, they can be printed like any digital picture.

On the downside, slide film is much less forgiving of bad exposure: when light is very contrasted, you have to choose if you sacrifice highlights or shadows (similar to digital), while negative film can take more contrast. It's also getting hard to find slide film in brick-and-mortar shops (I buy all of mine online), so when travelling you better bring enough film. Also, scanning film to have prints is more work, and good scanners for slide or film are more expensive than entry level scanners.