Well, it's not a straight apples-to-apples comparison.
You're right about isolating more than open headphones -- the Creative canal phones shouldn't leak sound out (unless you're playing music at ear-bleeding levels), and they will do a pretty good job of blocking out sound from outside. But based on fit and the type of tips used, the Shures, and certainly Etymotics, for example, would block out more outside sound, even to the extent of potentially serving as ear plugs without the volume turned on.
The best getting the Creatives would do in your case (in addition to providing a fun way to listen to music in semi-isolation) would be to let you determine whether you absolutely couldn't stand having anything in your ears ... if you don't like basic canal phones in your ears, you probably won't like the more expensive, fit-sensitive isolating ones. And liking the canal phones would be a good sign, but not definite proof that you would like the e500's.
Most lower-end canal phones use a very soft silicone tip that "scrunches" into the ear without much effort -- fit is important to the sound, but total isolation is not really the goal. With the higher-end IEMs; there are issues concerning whether flanges, floamies or comfort tips provide the best combination of fit/isolation, comfort and sound.
I don't mean to make it sound complicated -- it's not brain surgery, but losts of posters on these forums have expressed frustration with the process -- everyone's ears and preferences are different.
But you're right about the cost of getting other Shures to see if you might like the more expensive ones -- and the Creatives would not be "throw-away" 'phones, anyway. You would no doubt find use for them, even after being spoiled by really high-end earphones.