Xylitol vs. Sorbitol.
From a standpoint of chemistry, sorbitol and xylitol are fairly similar compounds. Both are classified as sugar alcohols (or more formally "polyol's"). And, as their classification suggests, both can be used as a substitute for table sugar (sucrose).
Of the two, sorbitol is the more widely used. It's common to see it listed as the sweetener in sugar-free products, especially chewing gum. One reason it's so popular with manufacturers is its cost. Sorbitol is less expensive than xylitol.
Sorbitol vs. Xylitol.
Sorbitol does not share the same effectiveness in preventing tooth decay as xylitol. While consuming sorbitol-sweetened (sugar-free) products instead of regular ones can play an important role in reducing a person's decay rate, generally speaking sorbitol has limitations.
a) Cariogenic bacteria can digest it.
Unlike xylitol, cariogenic bacteria (the bacteria that cause tooth decay) can ferment sorbitol, although they do it very slowly. That means, technically speaking, that it does promote their growth (although minmally) whereas xylitol does not.
b) Acidic waste byproducts are still produced.
The digestion of sorbitol can, at least in theory, result in the formation of acidic bacterial waste products that might lead to tooth decay formation. (In the case of low to moderate sorbitol consumption, the quantity of acid produced should be of little clinical importance. In the case of a person with impaired salivary flow, however, the risk of sorbitol-initiated decay might be significant.)
c) It's only low-cariogenic.
Because cavity-forming bacteria can ferment sorbitol (feed on it), it should be considered to be a low-cariogenic sweetener rather than a non-cariogenic one (like xylitol is).
Using relatively moderate amounts of sorbitol-sweetened chewing gum, especially as a replacement for regular gum, can be a good thing. But, in terms of cavity prevention, it's not a better choice than the regular use of xylitol-sweetened products.
Products that contain both.
Because sorbitol is cheaper than xylitol, a number of studies have evaluated products that have been formulated with both (as a cost-cutting measure).
In general, blends of the two have been found to be more effective at preventing tooth decay than sorbitol alone, but less so than xylitol-only products.
Look for 100% xylitol-sweetened products.
When purchasing a product, make sure to evaluate its labeling first. Many are formulated with xylitol-sorbitol mixtures instead of just xylitol.
In most cases, these products are made by lesser manufacturers who want to be able to make a xylitol claim but skimp on it in favor of the cheaper compound sorbitol. It's quite possible that the xylitol content in these products is so minimal that they cannot provide an effective dosing.