Personally, I prefer Gecko. But I am biased, because I like how XUL-based interface allows extensions to directly change web browser's look and behaviour.

WebKit has big corporations such as Google (Chrome) [granted, Google has since switched to Blink instead of WebKit], Apple (Safari), Microsoft (Entourage), Blackberry (Browser since 6.0) and Opera (uses Blink aka Google's derivative of WebKit) interested in the engine, its development, and monopolisation of market share.

Granted, there is a very fascinating project about porting WebKit to Gtk+ https://webkitgtk.org/ and an existing BSD-licensed mobile web browser based on WebKit https://en.wikipedia.org/wiki/Nokia_Browser_for_Symbian . Midori is fascinating as well.

In my personal opinion, applications running on Maemo operating system should depend only on Hildon widget toolkit, not on raw Gtk+ (just look at Navit, tiny menus are difficult to navigate), or Qt (even though it tries very hard to look like Hildon), if only for the sake of consistent behaviour (and lower memory usage - no need to have libraries for many widget toolkits around).

At the same time, web browser should have front end (GUI) entirely separate from backend (rendering engine) which means that switching between Gecko, WebKit, NetSurf, or Blink may be possible. And make porting to different desktop environments less painful.

And I would go far enough to say that apps should be interconnected to the point that web browser, instead of trying to open a Youtube page just like any other page, would allow the video to be opened by Media Player (with Youtube plug-in), thus entirely disregarding advertisements, Javascripts and other problems associated with trying to open Youtube in a web browser. But that's only possible if a third party writes suitable extension for web browser.

Whichever way you go, do take into account that there is a variety of open-source engines available, and my criteria for choosing the best one would be: 1) light weight by itself; 2) capable of dealing with heavy weight websites when RAM is tiny, memory is limited, network is slow, and CPU isn't fast either; 3) capable of rendering HTML 5 and passing ACID tests.

Thank you. Best regards.
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Per aspera ad astra...