Several factors…
- Check their list of free ETFs… the free trades are only on a specific exchange.
- If not on the list, factor in the 2EUR + 0.03% ETF transaction cost + 2.5 EUR/yr for holding positions on a given exchange
- Then look at the volumes… higher volumes = lower spreads (avg. difference between bid and ask price)
- Then look at the currency. If it is in CHF, you won’t lose 0.1% on currency exchange when buying.
But all this might be just optimising to the 0.01 Fr level so don’t go overboard.
Personally, if it is a one-off trade and it is free on a certain exchange, I’d buy on that one. If it’s not a “free ETF” then prefer the Six (Zürich exchange) listed one in CHF unless the volumes are really low. If it is a regular monthly trade that makes up the bulk of my investments, I would do the calculations…