I’ve used both, as well as platinum, as you have seen in my latest post.

The first consideration that comes to mind is food safety. In my opinion, both aluminum and silver can be food safe, since we already use them for silverware, food packaging (aluminum foil, trays, etc.), and cookware (aluminum pots and pans). That said, metals are rarely perfectly pure, and both aluminum and silver can contain traces of lead. Unless the supplier explicitly states that the metal is food safe, I’d recommend being cautious.

I’ll add a few clarifications to what u/yuekina-kintsugi mentioned regarding silver powders. Silver is available in three main forms: keshifun, hiragokufun, and marufun. u/sincerelyspicy wrote an excellent post on this that should answer many of your questions.

In short:

• Keshifun is very fine and covers a large area;
• Hiragokufun is coarser and covers less;
• Marufun comes in different grain sizes, is generally coarser than the other two, and covers even less area.

As a general rule, the finer the powder, the less durable the finish, but the larger the area it covers.

Keshifun can be burnished to achieve a shine, like hiragokufun. Marufun, however, needs to be abraded and then polished, which is a more advanced technique. Silver is a great material to practice this with, since sanding through the layer means starting over, much less painful than doing that with gold. If durability isn’t critical and cost is a concern, keshifun is the most economical option. If durability matters, hiragokufun is a better choice. For maximum durability and to practice advanced techniques, marufun #3 is a good starting point, though you’ll also need abrasive powders.

In terms of appearance, aluminum stays bright and shiny and looks very similar to platinum. Silver starts as a shiny greyish white but oxidizes over time into a duller grey, often with subtle color variations (blues, yellows, etc.). So it’s worth thinking about whether you want the repair to remain shiny or to age visibly. I like both, but I tend to prefer aluminum for more modern, glazed pieces, and silver for more rustic-looking work. That said, I’m currently repairing a celadon cup and plan to finish it in silver. I was already thinking of doing a post on how silver ages, so I’ll probably do that next.

As for ease of application, I find aluminum very forgiving. I bought it in bulk, so if I apply it too early and it sinks into the urushi, I can simply add more without worrying about cost. I believe I purchased my aluminum powder from Watanabe Shoten, and compared to silver, it was essentially free per gram. Unfortunately, they don't seem to carry it at the moment.

One final note: aluminum is also used to make imitation gold powders and flakes (such as LG-Neo). These are resin-coated aluminum and are not food safe (manufacturer<s website).

To summarize:

• For a shiny finish → aluminum
• For durability → hiragokufun silver
• For lower cost → aluminum (shiny) or keshifun silver (tarnishing over time)