If you haven't been living under a rock the past decade (I suppose Stonehenge qualifies) you may have walked in on some incarnation of the famous GPL vs BSD flamewar. It's up there with the most famous flamewars (now *there's* a research question for a brimming sociology student!) of our beloved Internet society.
Both licensing models have been around for a very long time. I don't know which predates which, but it really doesn't matter. The spirit behind both licenses is very similar: free software is good. But they realize this idea in different ways.
In the GPL license you have the four freedoms: to run the software, to have the source code, to distribute the software, to distribute your modifications to the software. What this implies is that when you obtain the software, you have the *obligation* to ensure that these four things hold true for the next person you give it to. After all, someone had to go to the trouble of preserving these rights for *you*, so you have to do the same for the next guy.
The BSD license is different, because it gives *you* the right to distribute the software, but it does not oblige you to make sure that the next guy has any such right. Well, that's not really a problem, the next guy can ignore you and get the software from the same source that you did (if that source is still available). But if you change it and you give it to him, you can forbid him from passing it on.
So who is right? Well, the BSD camp is. The BSD is no doubt a freer license, it gives you the right to decide what rights to bundle with the software. That is much closer to the absolute meaning of "freedom" than the GPL. Alas, it's not "completely" free, because you can't remove the name of the software's author and replace it with "Leonardo da Vinci".
What the GPL terms "freedom" is actually fairly subversive, because it *forces* you to do certain things. Most people who are forced to do something call that a "restriction" rather than a "freedom". It's true that you have certain freedoms when you get the software, but if you want to pass it on you have restrictions, so they could just as well call it the four freedoms and the four restrictions.
Therefore, if we take the philosophical ideal of freedom to heart, even though both of these licenses promote free software, none of them represent freedom, and the GPL is far less free than the BSD.
Suppose you're a parent and you give your kid a candy bar and say this is for you and your brother, you can have half of it, and when he comes home give him the other half. Do you think that is going to happen just as you instructed? How confident are you?
Well, your intentions were good. You tried to ensure fairness. But we humans are scheming devils, aren't we? So our philosophy is a bit of an idealization, we just don't live up to it.
Is there some way we can find a measure of freedom that is good enough? The fact is that we live with a lot of implicit restrictions without worrying too much about them. If you tell your kid you're free to wear anything you want, eat anything you want, be anywhere you want, and do anything you want, except you can't burn the house down most kids would find that a very satisfying degree of freedom, despite the restriction. They would probably say well I wasn't going to do that anyway, all my toys would go up in smoke.
So what can we do about sustainability?
Freedom in its pure form is a wonderful thing, but it's not inherently sustainable. You can take something and compare it up against freedom and tell if it's free, but you can't use freedom to enforce freedom. That would be absurd.
The GPL model is sustainable. It offers freedom, but with the pragmatic twist that there needs to be some kind of force to keep the freedom in place. In that sense it could even be said to be more free, because the *accumulated* freedom over all people involved is higher than when one person has all the freedom and everyone else has none.
GPL freedom is isomorphic. If OpenOffice needs a way to open jpeg files, and the gimp already has code for this, OpenOffice can just take it. Then two years later if OpenOffice reads jpegs much faster, the gimp can take the modified code from OpenOffice and use it. Both parties have the same degree of freedom, and no freedom is lost along the way, the process is "lossless".
BSD freedom, on the other hand, is "lossy". If I get BSD code I have a lot of freedom, but the next guy doesn't. It's fairly well known that there is BSD code in Windows. And obviously, whatever Microsoft did with that code, they have no obligation to release their changes. So the code *was* free at one point, but it didn't *remain* free. Furthermore, even if they didn't change it one bit, if the original author is no longer around, Microsoft is still sitting on BSD code that is free for *them*, but it's no longer free for anyone else.
So what can we conclude from all this? Both license models make software free, but only GPL software is sustainably free. The BSD gives greater freedom, the GPL gives more freedom. Choose which one you value more.
For a more in-depth discussion see this essay, not only for itself, but also the many many references it contains to other relevant texts.
UPDATE: Alexandre Baron has written a French translation.