Some comments after discussing about it on irc:
-Image sequence could be placed before video, and selected by default. (For a few reasons, the most important one being to have a real lossless export format selected by default)
-If possible, maybe select the default image sequence depending on the image bit depth (png for 8bit, exr for higher bit depth). And then if user select another format, remember last selected format depending on the bit depth.

-One more note: the radio button, "All" is maybe too generic, what do you think about "Both" ?

After some comments from troy, I'm more convinced that selecting exr by default for any bit depth for image sequence export could be a good idea.

Check first if exr export actually works and whether ffmpeg can turn exrs into videos. Also don't forget that exr doesn't have thumbnails by default on many platforms' file explorer, so there's a chance we're going to get bombarded with questions about this strange file format.

I don't know if 8-bit images should be using EXR. While yes, they are technically superior to PNG, they are not very well supported out of the box. You need special software to even open EXR files and they aren't supported on the web. If Windows and websites natively support opening and viewing EXR files, maybe we can switch to that for the image sequence.

I submitted a patch with changes, but I didn't touch anything for now with image formats as that might lead to other bugs I don't know about.

Ok. Just remembering last setting used might be enough.
I agree with your comments, that's why I suggested to still have png by default for 8bit images.

Maybe we can consider 8bit images as "final consumer export", and higher bit depth as "production assets export", and save default/last used settings relative to that.

As for the question about exr and ffmpeg, it looks like ffmpeg has some very limited support for exr...
This page has interesting things to say on the topic: https://trac.ffmpeg.org/wiki/Encode/VFX
which makes me wonder, what should we do for video export of high bit depth images?

This one is relevant regarding the latter: https://bugs.kde.org/show_bug.cgi?id=375217

We can technically give exr support for high bit depths, as ffmpeg can do anything with exr it can do with png, but it just doesn't comprehend color management, so to do this right, we would need LUT baking to begin with, and let the user add a lut baking filter onto everything, or at the least warning them. But I suspect there might be an equal reason to just let people export their exr files and open them in natron or blender and let that handle the more advanced stuff like making sure the end result has the correct color format and the like.

It sounds like changing defaults might need a bit more discussion if it ever gets done. This ticket was mostly a GUI change, which was added to master already. I am going to close this ticket out. If there needs to be dedicated discussion about export formats and default, that can probably be done in another ticket to keep it more focused.