An under tank heater is by far better than any heat lamp. Leopard geckos are nocturnal (or crepuscular depending upon your view) and thus do not bask like diurnal lizards like say, a bearded dragon. Heat lamps work best to create a "basking spot" which simulates how a reptile would bask in the sun when in the wild. However, since Leos are not diurnal, they do not "bask" they spend the day hiding under rocks and logs and absorb heat from the ground there, as well as lying on rocks which retain heat after the sun sets. This is one reason why they're better. Secondly, leopard geckos need 88-92 ish degrees Fahrenheit to properly metabolize and digest food. A heat lamp will heat the ground beneath it, but not to the necessary temps like a heat mat would. The only sources that say you need a heat lamp are generally inexperienced Leo keepers, pet shops (which don't have proper advice anyway), and highly experienced breeders and keepers (which have the experience and expertise to use lamps carefully). Heat mats are also cheaper. They're an upfront cost of about $30-$40 and will last years if properly maintained. Heat lamps on the other hand can pose fire hazards and need to be replaced every few months, and a good, powerful bulb will cost you about $12-$20. Besides this, Leos have sensitive eyes and don't like light very much (even infrared bulbs won't help that much). It's just easier and simpler to get a heat mat from the start and save the time, trouble, and money.
