As Weblate 1.0 is out, it's time to compare it to existing solutions. As first I've chosen Pootle as this is tool which we have been using for quite some time and I used it as an inspiration for Weblate.
First of all, both Weblate and Pootle use same backend for manipulating with translation files - Translate Toolkit. On the way I've learned that I've chosen different way of manipulating with backend files, what seem to lead to much smaller changes (no more reformatting of whole po file as Pootle does). Also Weblate does not suffer from some Pootle issues like not properly handling changes to fuzzy flag, which has bitten us several times at phpMyAdmin.
Another shared thing is web framework Django. For me it was obvious choice as I like writing application with it. The biggest difference here is that Weblate relies on Django admin interface for maintenance, while Pootle has everything in it's interface. Pootle has great advantage here for having consistent interface for everything, what was sacrificed in Weblate for less code to maintain (while having powerful Django admin interface). Also sharing same framework allows quite easy migration of user accounts.
Now let's functional side of things - this is where I've tried to take all good things I've seen in Pootle, while adding features I've missed there. What makes biggest difference here are project/subproject classification in Weblate, which was originally started as way to handle more project branches, but proved to be useful for translating related projects as well. This allows automatic propagation of changes across project, what is really great for translating more branches of same program or different interfaces to same functionality (eg. GUI application and command line tool).
The other big difference (and I would say key feature of Weblate) is Git integration - all changes are commited to Git with correct authorship, it can help in merging po files or automatically pull upstream changes (eg. from GitHub). This all makes integration into (Git based) development process really easy.
Other features like consistency checks or dictionary (terminology) are quite similar, though different. There is more consistency checks in Pootle, but the main reason was that I found some of them too annoying. User dictionaries are still pretty basic in Weblate and that's definitely something what should improve in future versions.
On configuration side, Pootle is much more flexible. The main reason here is that I've never used most of the options there. So there are no per project settings of checks or privileges. There is also no way to add language from Weblate - usually adding translation is not just matter of adding appropriate po file, but also other steps, which anyway had to be done manually. So Weblate leaves this up to user. On the other side as soon as new language pops up in Git repository, it is shown in the interface.
Both projects have quite good documentation - Pootle has most things covered on wiki while Weblate relies on separate documentation. However there are definitely some bits missing on both sides.
PS: If you want to want to have more "facts" in comparison, you can check Ohloh :-).