A Dental onlay should always be the first treatment option for significantly decayed teeth. However, placing a dental onlay is technique sensitive and requires skillful hands. Both treatments are good options and will protect the tooth from further damage.
A dental onlay can last 10-15 years, sometimes, even more, depending on how you care for it. They are exceptionally durable and long-lasting.
No, a well done and well fitted dental onlay should not hurt at all. You might feel some soreness in the tooth, and surrounding gums after the anesthesia wear off. You may also feel sensitive to hot and cold for a couple of days after the treatment. All these things are normal and will go away in a few days.