A perfect question just in time for holiday cocktail season! And what a great idea to avoid plastic.
There should be no bisphenol-A in unlined aluminum ice cube trays (there has been, however, BPA in epoxy linings of aluminum reusable water bottles — the company SIGG got in hot water over this a few summers ago and has since changed the lining they use to something else that is also proprietary, if I’m remembering correctly).
The issues with aluminum are that it isn’t durable, it’s not great environmentally as it is energy intensive to extract from the earth (though the components of stainless steel aren’t exactly energy neutral either), and there have been lingering concerns over the years–largely unsubstantiated but enough to warrant multiple studies–re links to Alzhiemer’s disease.
The long and the short of it is that I prefer stainless. The price evens out as you will probably have to buy five rounds of aluminum trays before your stainless shows any wear and tear. I’m not sure if aluminum is recyclable where you live, but that’s also something to consider. Stainless also goes easily into the dishwasher without any rusting (a big issue with flimsier aluminum). What else can I say? I’m big on the Precautionary Principle — if the risks of aluminum are unsubstantiated, what’s 23 bucks to err on the side of caution? Stainless steel is considered very safe and won’t leach chemicals into your ice.
Hope this helps. And cheers!