Confused, need help

Hi peeps, my joining in Deloitte is 28 October and i am receiving dstart mail to complete new hire tasks. I am not sure whether i am gonna join Deloitte or not. What i should do in this case. Should i proceed to fill all the details like bank account details n all or just ignore it. The case is i am having another offer which is like an startup. But the work culture will be far better than Deloitte( i have confirmed everything with so many employees of that org so saying). Please guide me.