Haha, pick ya poison, huh? I pick Canadian rye whisky
Chlorine is also a nutrient, and is essential to life. Copper is also a poison, and copper is also essential to life. Silver is not known to be essential to life, but is a known poison. Not that it makes any difference because we're talking about pools here, and almost everything "poisonous" requires a comment about dose to complete the sentence.
For example, chlorine is not poisonous at levels used in modern potable (drinking) water systems, nor in swimming pools maintained at levels sufficient to control pathogens and algae.
But I have to ask a question too. Why not just use modern water disinfection rather than something ancient like copper and silver?
In a residential pool, you can use anything you want. You could refill it every day from a city water system, although until the sun hits the pool, it would actually have more harshness from chlorine than if you maintain it properly. You could boil it in a big kettle if you like, or sprinkle it with potpourri. But if you operate a commercial pool in most jurisdictions, you're required to maintain the same rate of approved disinfectant (chlorine, bromine or biguanide) whether you have a penchant for metal ions or not. Thank goodness the world has organizations like WHO, CDC, AVPMA and others who have debunked many of the bogus claims made by manufacturers of ionizers.
The downside is the potential for metal staining (strictly a cosmetic issue, again, up to you), so it's easier to just use chlorine and get the job done - minimize disease transfer and keep the water clean. Unfortunately (or fortunately, depends on your perspective), silver and copper are not very good at preventing disease transfer, while they are better at preventing algae. A properly run swimming pool has algae as an early warning sign that sanitation level may be getting closer to inadequate.