If you're the type of person who prefers immediate results, consider having a professional do the work. Also consider having a procedure if you have a condition that requires close monitoring such as receding gums.
Dentists use whitening products that have higher concentrations of hydrogen peroxide, the primary bleaching agent. Because they're more concentrated, many of these products produce noticeable result after just one session. But the higher concentrations also make these whiteners harsher which increases the risk of harming delicate gum tissue and teeth.
That's why professional-grade teeth whiteners should never be used by anyone who hasn't first received proper training. Professionals know how to take appropriate precautions against such harm so that you get the results you expect without experiencing any adverse affects.
When you have the whitening procedure performed in a dentist's office, the dentist likely will apply a solution directly to your teeth for about 45 minutes and they may or may not use a light source. Some dentists add light to the whitening process as they believe that doing so helps teeth whiten even faster.
The only real drawback to having teeth whitened by a professional is the cost. You're taking up more of the dentist's time and for that, you'll have to pay. Teeth whitening done in a dentist's office can cost upwards of several hundred dollars or more for three 45-minutes sessions. Check to see if they have affordable financing. Just understand that because it likely will be considered a cosmetic procedure, your dental insurance may not cover teeth whitening. So be sure to check first.