The following list is very generalised; of course, you must weigh up the differences between your specific opportunities:

1. What industries are they in?
2. What is the role itself?
3. How large is each company?
4. What is the company culture like?
5. What is the structure of the company in terms of hierarchical-ness and number of employees at each level?
6. What is the reputation of each company?
7. What is the background of current employees?
8. What is the investment into training and development at each company?

The pros of joining a grad scheme at a big corporate firm:

1. Money - your salary will probably (but not always) be higher and since it's your first job you probably don’t have savings to tap into to move to a big city. Making money is objectively important and so you should think carefully about what will give you what you want in life.
2. Security - you generally know what you’re entering into on a grad scheme in terms of the expected tasks and roles. You won’t be asked to stray too much out of your prescribed role, while at a startup it’s likely that you’ll need to perform roles across multiple divisions.
3. Exit opportunities - a large company will give you a good brand name on your CV and signal credibility to future employers. This ensures that you have a greater number of opportunities available to you after completing the graduate scheme, reducing the risk of finding a new job if you don’t like the graduate scheme.
4. Training - big firms have a lot more money to invest into training for employees than startups. You are likely to have an extensive training period at the beginning of the graduate scheme, ongoing training sessions every few months, and online courses or training that you can take at any time. Meanwhile, at a startup, you will need to be a super-fast learner and pick up things by yourself because it is unlikely that other employees will have time to teach you.
5. Networking - a larger firm will have a larger alumni network. They are also more likely to maintain their alumni network. You can use this alumni network to secure opportunities in the future. You will also be able to get to know more people at all different levels in the firm.
6. Corporate benefits - on grad schemes, you tend to be treated very well, particularly at companies with a higher revenue per employees. You could expect to have numerous fine-dining experiences, travel business class around the world, and get tickets to free events across the city. Moreover, you may have opportunities to transfer offices, and do client secondments (i.e. work internally at a client’s office for a few months).
7. Work-life balance - the hours on a graduate scheme are more likely to be standard and predictable. You’re also less likely to have to work weekends.
8. Making friends - on a graduate scheme, you’re probably joining at the same time as a lot of other people that just graduated from university. HR is likely to organise a lot of social events at the beginning for you to get to know your joining cohort. At a startup, you could be the only employee your age.

The pros of joining a startup: