There's no "best" only best for you.
Fund availability differs by service provider as do management charges. Fund preference will depend on your risk appetite. Generally more risk = more potential reward as well as more potential loss, and you need to be comfortable with risk of the worst case scenario.
Contact both service providers and look at the fund offerings for the relevant account. Note that it's not always clear which funds are available for which. Once you identify your preferred fund for each (and they probably have near identical offerings for the common indexes) just go with the one offering a lower management charge.
I personally went for a 50-50 nasdaq 100-s&p 500 split with Zurich. That was best for me, but it could be awful for you- nasdaq 100 is crazy volatile so i can well understand people avoiding it despite the significantly higher long term returns.