Why the bowl keeps losing this argument
Rats dunk. They carry food to water, dip bedding in it, wash their faces in it and occasionally sit in it — so an open bowl left in the cage is rarely clean water for long. Within an hour or two it can hold a cold soup of softened food and bedding fragments, and that mixture grows bacteria fast at room temperature. A rat may still drink it, but "will drink it" is not the standard to aim for when the alternative keeps water as clean at six in the evening as it was in the morning. For a committed dunker, the bottle is not the lesser option; it is the only one that survives contact with rat behaviour.
Making the bottle work properly
A bottle's whole job depends on one moving part, so set it up carefully. Hang it outside the bars with the spout at a height your rat reaches by sitting up slightly — low enough for comfort, high enough that bedding cannot pile up against the tip. Fill it fresh daily, and every time you hang it, tap the metal ball with a fingertip and watch a droplet appear: that tap-test proves the valve is flowing and not air-locked or jammed with a crumb. Give the bottle and tube a proper scrub weekly, including the inside of the spout with a small brush, because a slimy film builds up exactly where mouths touch. If you keep several rats, one bottle between a few rats is wise — a single jammed bottle then never leaves the whole group dry.
If you keep a bowl as well
Some rats genuinely prefer lapping, and a bowl can stay as a second source if you place it to fail gracefully. Choose the heaviest ceramic bowl you can find, stand it on a raised, washable platform away from the main bedding area, and accept that it needs emptying, washing and refilling at least twice a day — more in warm weather. Offered like that, a bowl is enrichment and choice; left on the cage floor under a food stash, it is a hygiene battle you will keep losing. Never let the bowl be the only water while you are away overnight: the one night it gets tipped or fouled is the night that matters.
Watching that everyone is actually drinking
Either setup needs a watcher. A falling bottle level each day is reassuring; a level that stops moving means the tap-test starts immediately, and if the valve is fine, the rat who is not drinking needs attention. Drier-than-usual droppings, a quiet rat huddled away from the group, or food left untouched alongside an undrunk bottle are reasons to call your vet the same day rather than wait — small animals feel water loss quickly, and a rat off his water is rarely a rat who will fix it himself.
Putting the daily bottle check and refill on your rats' KinsLe Care Teams duty list, marked complete with a timestamp each morning, means the one job that keeps their water safe never depends on someone remembering whether it was already done.