Either heating method will work with leopard geckos. Some under tank heating pads can short out or overheat but this seems to be more rare than common. Leopard geckos don't actually bask, being nocturnal reptiles. However they do need a heated area of the tank around 85F I beleive it is. (might be 88F, leopard geckos are not my most familier gecko. Fortunately there are plenty of good caresheets out there for them)>
Having a basking light shinning on a couple flat rocks during the day and letting it go out at night, will provide a warm place for your gecko to warm up in as well. You can also have both, the basking light on during the day with supplimental heating at night with the under tank heater set on a thermostat/timer. You still want a cooler area of your tank for your lizard to go to as needed.
Now, gut loading crickets.
Potatoes and carrots are not sufficient to gut load crickets. They do provide excellent sources of moisture and some other nutrients. The best thing to feed crickets is mixed greens (greens such as dandelion greens, collard greens, endive, escaroli, squash, bell peppers, etc). Fish flakes, low fat dry catfoot (crushed) skimmilk powder and grains (wheatbran etc) added to the fresh vegetables also provide a good gutload. Adding a bit of multivitamin and calcium powder to the dry mix (be it just fish flakes, or crushed catfoot/skimmilk powder) will also ensure more vitamines go into your crickets.
I find it best to feed a variety of foods to your crickets, with alot of fresh foods (which also double as moisture sources), with fishflakes and a pinch of multivitamin/calcium powder mixed in, for dry food. If you have any vegetarian or omnivore animals (ie green iguanas, bearded dragons, uromastyx, guinea pigs, rabbits etc) you can give the same assortment of greens they eat directly to the crickets. Bearded dragon or green iguana pellets also work for crickets (though since these are dry, you will need to provide a moisture source, such as potato, squash, carrot, slice of orange or other moisture rich vegetable (watermellon works good to).
For other feeder insects such as mealworms and superworms, doing research on how to breed them will include how to feed the worms healthly.
