Here's some advice. First, when purchasing a toothpaste for you or your child, select one that contains fluoride. Fluoride-containing toothpastes have been shown to prevent cavities. However, one word of caution: check the manufacturer's label; some toothpastes are not recommended in children under age 6. This is because young children swallow toothpaste and swallowing too much fluoride can lead to tooth discolouration in permanent teeth.
Next, when considering other properties of toothpaste -- such as whitening toothpastes, tartar-control, gum care, desensitizing, etc. -- the best advice for selecting among these products may be to simply ask your dental hygienist or dentist what the greatest concerns are for your mouth at this time. After consulting with your dentist or hygienist about your oral health's greatest needs, look for products within that category (for example, within the tartar control brands or within the desensitizing toothpaste brands) that are readily available.
Finally, some degree of personal preference comes into play. Choose the toothpaste that tastes and feels best. Gel or paste, wintergreen or spearmint all work alike. If you find that certain ingredients are irritating to your teeth, cheeks or lips, or if your teeth have become more sensitive, or if your mouth is irritated after brushing, try changing toothpastes. If the problem continues, see your dentist.