The choice of whether to work for a start up or an established MNC player depends on many factors including your outlook and approach to the job, risk appetite and aims/goals.
What most people do is to weigh the key pros and cons of both.
For instance, when you work for a start-up, the job description is flexible, so are the working hours, you will be doubling up and covering other jobs too from time to time. Many start-ups do not have a dream run and growth, such companies run into financial difficulties, this would translate to a stressful work enviornment, lay-offs, delay in salary/allowance payments, backlogs and hold on promotions and increments.At times, if the company folds-up, then you would loose the job. Office ambience, work culture, office space all would be modest. Many people dream of a quick trip overseas once they join the job, this may not be feasible in the start-ups.
Let's consider the well running MNCs, it has brand value, social recognition, adds to your image ( I work for Infosys!), The work culture is more or less standardised and the teething issues are long gone. These companies have a formal greivance redressal policies and protocols, including abuse and sexual harassment. The number of employees would be more, hence the PF, allowances, healthcare benefits and travel arrangements would be much better. In the long run when the markets fall or the industry runs into a crisis, the established MNCs would be better placed to weather the storm in comparision to start-ups
The unique appeal of start-ups is for the passionate graduates and young minds who are eager to take the risk of an unsteady enviornment and career growth in exchange for the valuable knowledge gained with regards to how to set-up and run your own firm or company. Minds that need constant challenge and good with leadership and trouble shooting would tend to join start-ups.
In the current difficult job market, with 6 months of internship in the start-up, I would lean towards the job at Infosys for more stability at this point of time.
Before you make your choice, kindly check your contracts with the start-up with regards to the clauses related your discontinuation and refusal of job confirmation.