I can not say anything about the politics behind both projects, but here is what I think/have found out about both the projects:
Development Activity: Both projects have pretty much same amount of commits in last couple of months. As for quality and impact of each commit, I can't say much without checking the code, but from commit messages it seems to be quite similar too. [pentadactyl is always one or few version behind the Firefox releases]
Usability: For average user, Vimperator seems much easier to use, while Pentadactyl has a steeper learning curve but would feel very similar to vim users[1].
2.1. Keymapping: The options and keymapping in Vimperator are much easier to remember for average users, for instance, to show menu bar, in vimperator you can simply :set gui=menu
(substitute 'navigation' for navigation bar, 'tabs' for tab bar etc.) whereas in pentadactyl it is :set go+=M
or :set guioptions+=M
(a single character for each item which is also case sensitive)
2.2. Customization: On the other hand, you can customize a lot more options in Pentadactyl then you can do in Vimperator. Taking our previous example, in Pentadactyl you can turn on and off bookmark bar, status bar, side scrollbar, bottom scrollbar and lot more using the same method as mentioned above, whereas Vimperator supports much less customization.
Design: Vimperator has a much more modern look than Pentadactyl.
Depending on what is important to you, both projects are good, if you don't care about aesthetics too much and are willing to spend some time learning the exact commands (or look them up in help, which is pretty simple to navigate) but want lots of customization options, or if you are looking for more vim-like experience, Pentadactyl is for you.
On the other hand, if you are an average vim user who wants to turn their browser experience to keyboard use only but don't want to spend too much time learning Vimeperator is for you.
[1]: Basic functionality is similar in both, like open pages, going back in history, closing tabs etc. it's the other features that I am talking about.
[2]: Vimperator may have options hidden deep down to enable/disable other gui options, but even if they exist, they are not as visible as Pentadactyl.
12@suprjami He has criticized specific points, about their philosophy. He's not saying they are evil for forking, just saying that their philosophy is wrong (from his POV). What's wrong with that? – 0fnt – 2014-11-07T11:22:34.133
1@0fnt I would guess most people would expect answers of a rather encyclopedic nature in a stack exchange website. He is not saying the philosopy is wrong from his POV; instead this is plain sarcasm. – geo909 – 2016-05-11T13:13:56.757
2You guys have not parted well, eh? ;) Well anyways - thanks for the cool plugin - I've just switched it definitely feels weird upvoting you and then commenting on this post by just using keyboard. Now fAdd to add comment ;) – nikib3ro – 2013-09-18T20:03:59.407
37This answer is unprofessional and makes you look bad. If someone forks your project they are not an enemy for you to openly criticize, they just have a different idea than you do. You should present facts and let users decide based on their experience, don't slag off your sister project because they put in a few commits you don't personally agree with. (NB: I just discovered these plugins last week so I have no affiliation to either project) – suprjami – 2014-01-18T21:47:10.027