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Yeah - I haven't been able to get my hands on one in order to do a proper comparison yet, but I'll give you my initial thoughts anyway.
As far as timbre goes, they're a lot closer than I expected. The slide whistle can be a little reedier and airier than regular whistling (though the airiness of a whistling sound varies on the individual whistler). It does have a somewhat more "comedic" quality to its sound though, so you may need to pay attention to how much you use it so as not to distract the audience from the piece has a whole.
I was originally thinking that range may be an issue, but upon listening to this (https://youtu.be/poysoABw-hU?t=35s) I discovered that they have a range of around 3 octaves. TIL.
Something that you'll want to watch out for though is the difficulty of intonation - as far as I know they don't really have any clear-cut defined harmonics, and can lose tone and pitch in the extremes (especially the upper) of the register. This could likely be improved with good breath control but would probably require more effort than it was worth.
It's also difficult to exactly pinpoint a certain pitch as I imagine the position would move around depending on the weather and humidity of where you were playing affecting tuning.
It'd also be considerably more difficult to do large jumps in pitch, as the slide requires much more physical motion than just a slight alteration of the embouchure such as with regular whistling. This combined with the fact that you'd have to actually cut off the airflow, move the slide, and then restart the airflow again in order to get in between two notes without a potentially unwanted glissando could also make playing difficult.
The verdict?
It /could/ work, but it wouldn't be easy. Good luck to you if you try it, and if it doesn't work out then slide whistles are only like a dollar anyway and they're great for a myriad of other things such as irritating your neighbours, roommates, parents, friends, lecturers, etc.
Oh lol by Western I thought you meant like not Eastern, rather than like Wild Wild Western. Listening to this example I'd say that a slide whistle probably wouldn't really cut it, unfortunately :/