The GPL is a better choice if you want to make money from the software. With a pushover license, your competitors can extend the program and profit from it in a way you can’t because they aren’t required to give the changes back. The GPL evens the playing field. Of course, you often see the original company requiring a CLA so they retain copyright over all of the code.
On the other hand, it does enable possibilities that you would be very unlikely to get otherwise. For example, Cedega (formerly WineX) forked Wine when it used a pushover license and brokered deals with game companies to make the DRM compatible with WineX/Cedega. That meant you could play these games on Linux-based OSes with Cedega, but not Wine. I really wonder if it would have been possible to make Wine compatible with some of these DRM schemes otherwise. Consequently, however, Cedega could not incorporate any changes from LGPL’d Wine, as that would have required them to license Cedega under the LGPL, too.
That’s another issue. You can incorporate MIT-licensed software in GPL software, but you can’t incorporate GPL software in MIT-licensed software. So going with the GPL gives you more options. As SerenityOS is building everything from scratch, this isn’t an issue, but you can well see how it could be. The LGPL is far less disruptive to people who want to release their software under a pushover license. It only requires you give back any changes to the LGPL-licensed part, and does not cover other parts of your program. Personally, I really like the LGPL. It levels the playing field while being quite compatible. It’s not perfect either, of course.
It’s a tricky question, and there are no right answers. Ultimately, the decision is up to the developer and I can’t fault any choice, including the decision to use a proprietary license.