Trending @Wipro; Confusion over new opportunity

I have been working on wipro from 3.5 years started as a fresher with elite package as dotnet developer. I have got the offer from one small company founded in 2012 and they are giving package around 8lpa and in wipro next year in by march my salary will be 6.7lpa. The catch is they are not giving offer letter they will only provide offer confirmation I got this opportunity through referral. I'm confuses whether to accept and resign in wipro Or not. Please suggest.