if u need more grip,use soft inserts.u will get the most grip tis way,but do take note tat this is at the expense of ur tyres' ;ife span.
here's a rule 2 remember :
soft inserts = best grip,but worst life span for tyres
hard inserts = poorest grip,but will prolong the life span of ur tyres...
wat 2 use will depends on ur car's setup also...