Honestly a glass tank is not a good setup for a ball python period, that's why it's nothing but hassle to keep your temps steady and humidity right, you end up having to use heat lamps and kill airflow, it's just the reality of trying to maintain reptile needs in a fish tank. Personally I'd scrap both and get yourself some pvc cages with radiant heat panels and flexwatt underneath. Your first setup is better temp wise and the experts advice sounds insane to me, water and pebbles why? Undertank heaters don't affect your ambient temps so unless your room is above 75 it's not really feasible to run tank or tub with only belly heat. What kind of thermostats controlling the UTH?