I've done both GSoC and internships, and I don't think they are easily comparable from this angle.
An internship teaches shows you how companies function, lets you work in close proximity with your team, be part of a process, and learn. You get to interact with seasoned veterans that can mentor you along the way and show you how to get better, which is invaluable. It can be a good or a bad experience, but you'll learn something either way. I found it helpful to start out doing internships, because there's more guidance provided, people acknowledge the fact you are pretty new to programming. It's a two-way experience that you might not be able to obtain, purely, from GSoC.
On the other hand, you can't succeed in GSoC without self-discipline and self-motivation. This shows what kind of control you have over yourself, whether working remotely means you slack off, and how passionate you are about programming. It's important to companies because it reflects your work ethic. Additionally, the code is in the open, and it can serve as a testament of your skills at a certain point in time. The mentorship/guidance you receive is of a different nature, but the emphasis is on you. I appreciated the considerable freedom I had in terms of certain decisions during my GSoC project. I think it's a great way to prove yourself (to yourself, firstly).
There's a general opinion that open source contributions help with potential jobs; Google themselves mention that participation in GSoC helps if you apply to work for them. I think the consensus is that, by contributing to open-source, you show dedication as a programmer. (It probably helps even more if you kept contributing to whatever project you were on during GSoC, after the summer ended. I'm not in that position, but I would certainly ask a potential hire about why they did or did not continue working on their project.) You show an interest in your skill that goes beyond a paycheque, and all things considered companies prefer that. I certainly enjoy working with someone that codes it because they like to much more than I do with someone that is just trying to get rich.
In conclusion, I think GSoC is valuable, not because Google sponsors it, but rather because it's an open-source endeavour that reflects well on your interests as a programmer. I think internships can help you become a better programmer and a much better team player, and they offer a different kind of learning experience: you learn how FOSS development works in one, and how companies do in the other. Practice a balanced diet.