From my experience with shotguns, it comes down to accuracy. A smoothbore barrel won't be too accurate out past 50 or 75 yards I don't think. If you get a rifled barrel and try out some of the newer sabots, I think you will find you can hit a paper plate (about the size of a whitetail deer's vitals) at 150 yards without too much difficulty. The slug is starting to drop pretty fast out there at that range, so practicing at 100, 125, and 150 yards would be important.

Regular smoothbore barrels are not that good at stabilizing slugs, especially sabots. A sabot round needs the spin from the rifling to allow the sabot to separate from the actual bullet. Sabot rounds are what you really want to use for deer, they are way more advanced than the older slugs.