no world etf or fund at any Swiss broker will beat VT at IBKR 
VT at IBKR is cheapest (to buy, to hold or to sell), diversified and without stamp duties. By the way, you can also buy FWRA at IBKR.
So the reason to choose another broker over IBKR will not be financial. It would be more about familiarity , US estate tax understanding etc
Read this thread for some more numbers Here
Last point -: at some point you will exceed the threshold for US WHT tax credit and then you will be able to get the money back.