float switches can be more reliable as they offer redundancy. For instance you can wire up as many switches in series as you want with each switch slightly higher than the next so if one were to fail the next one will take over. You can also wire in a timer to not only control when the top off activates but also control how long it's allowed to activate for. However such features are more critical when a larger or unlimited source of water is used for top off. If it's a small container that can't physically add enough water to cause damage a simple mechanical float is all you need, provided it can gravity fed. If you need a pump then you'll obviously be better off with a float switch.
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