Both are C++: developers are more expensive to hire, and stuff takes more time to develop and debug. I'm proficient in C++ (programming for living since 2000), but I wouldn't pick the language for 2D GUIs in 2019.
> GTK+ can be programmed in Vala
Picking a non-mainstream languages is risky. Harder to find developers. Way more likely to find bugs in the toolchain and these can easily cost weeks to fix.
Have you seen the state of Javascript 'ecosystem' and job market?? Both statements are false.
The real reason is that C++ toolkits don't have platform and device independence. HTML and CSS are really good and powerful design tools, especially compared to composing raw pixels like you'd (essentially) be forced to do with Qt and GTK.
Let me correct: GTK+ is not C++, it's C. Qt is very cross-platform: it supports about 10 platforms, including Linux, Windows, and macOS. You are not doing anything close to compositing raw pixels in Qt or GTK+. They have layout managers that are specifically made for typical application UIs, and they have lots of ready-made widgets. Qt also has QML, which is a declarative UI descriptions language kind of like HTML/CSS but much more productive because (gasp) it's actually designed from the ground up for applications, not websites.
Which one would you recommend?
Cross-platform story is complicated, though. I have good experience creating a custom GUI framework for embedded Linux: drm/kms, gles, NanoVG, [DllImport], everything on top is custom C# code in .NET core 2.2. But that approach is not for everyone. I only needed 10-15 screens with simple GUI on them, it’s a 5” touchscreen with no other user-facing devices. Creating more advanced UX this way would consume too much time.
There’re Avalonia and Xamarin. Didn’t used them at that time because I needed full-screen GUI for a single-application Linux, without any desktop environments or user mode OS components. For desktops and especially mobile apps they might work OK.
It’s unpopular opinion here, but I think Electron is not that bad for GUI, esp. when writing TypeScript instead of JS. I think it’s similar story to Unity3D or PHP: low entry barrier attracted inexperienced programmers, then the whole ecosystem is judged based on the output of these inexperienced people.
Qt, as well as Windows, macOS and Linux actually can do Android [0] and iOS [1], and Web is halfway there [2].
[0] https://doc.qt.io/qt-5/android.html
You know all of the arguments against Electron that people trot out about how it's not real native, and it doesn't follow the right UX conventions or paradigms? Qt on the web is exactly like Electron on native, except way worse and without any sign that it's going to improve, because fundamentally Qt is structured around the idea that it should be able to spit out a binary blob of pixels instead of hooking into web-native primitives like the DOM.
Look into something that's showing more promise, like .NET and Razor. The Rust community is also doing some good work here, although I don't know that they have any UI frameworks that are mature enough to be reliable in an enterprise setting.
These aren't true cross-platform efforts yet, since they're still forcing you to think about HTML. But if I was going to bet on any project ending up with a real cross-platform solution that felt good on the web, some kind of export option from an Open Source Windows Forms or similar is probably what I would bet on. Either Microsoft or (possibly) the Rust community are the two parties that I think are most likely to end up producing a true native UI toolkit that can actually export to the web in a usable, acceptable way.
[0] http://www.beeware.org [1] https://dan.yeaw.me/posts/gui-widget-for-beeware/