If you're doing an internship for your CV and nothing else, do a summer placement. If you're doing an internship for yourself, do a year long one.
Advantages of doing a year long placement:
- Projects are more likely to be relevant and valuable to the business. Simple fact: even small projects can't be fitted into a 12 week period. It is common for companies offering summer placements to give their interns projects that have already been done before or don't impact the business because they can't afford to give them a better project when there is a decent chance they won't finish it/ leave lots of loose ends.
- Better integration into the company. People will find more time for you when they know you're not going to be gone in less than three months. If you're there for the year, you're less likely to have that "just the intern" mindset.
- Better idea of what you want from your career. This is important. It is a lot better to do a year at a company and decide it's not for you rather than reach the same conclusion after committing to a 2-4 year graduate scheme. All the people I know who have finished a year in industry have offers from the company and know for certain it's what they want, or they don't go back but have a much clearer idea of what they want to do when they graduate.
- Better employment prospects. This is a result of the first two points. Having a household name on your CV will be helpful for getting past initial screenings but someone who did twelve months is more likely to perform well in the assessment centres than someone who has done three, simply because they have four times as much experience.
- Wages will increase a lot faster for graduates who have done a placement year. Their experience will mean they hit the ground running in the graduate schemes while their less experienced counterparts are still finding their feet. Accounting for graduating a year later, there's not much difference financially between those who have done three month and twelve month placements.
I'm biased but I hope this helps.