The string will give a reasonably accurate indication of the relative airflow over the fuselage and is often preferred in the slower speed bracket whilst the balance ball will give an indication of skid in the turn but will only remain central on a constant heading if there is a little opposite bank applied to counter the inherent sideslip.
This is my experience.
'Ball' in my view must be considered more reliable, especially in low speed regimes where the rotor downwash affects the strings.
The string only starts to flutter in
very slow speed conditions ie. nearing hover but as crab mentioned is often more reliable at "slower" ie. between 20-50kts than the slip indicator.