Let's try point by point:
Why (Mobile First) or (First Desktop) better ;
Why (Mobile First) or (First Desktop) worse ;
something from the "What is better a spoon or a fork?" You know... It's only a question of convenience, and it depends on what your original template, and in what way it will be easier to adapt. But even in this case, the word
here a little is appropriate, rather it is a matter of convenience. We (in total, people 50+, from students to experienced developers) on the basis of the number of parameters marked "to do so" from Google PageSpeed spent a lot of tests and discussions... as a result of which a single proper approach has not been found. The most convenient from the point of view of common sense, was this:
1. Please write all the common styles, describe the colors, fonts, and all such nonsense
2. For each range of permissions create your style-corrector, which describes the (correct) positioning of elements, their size, font size, etc.
So, as You know, "first" generally disappears as such, it turns out "and". But Google do not like this approach, he asks to put "all the important styles are" in , and to separate in this mode, the "major styles" from the "not important" is impossible, since the degree of "importance" is determined depending on the device resolution. But speaking only about personal convenience - it is more convenient to start with the mobile version, because it is a priori "less" and expand the item much easier than "sculpt a smaller version of it".
Performance (Mobile First) or (First Desktop) ;
The performance is not affected in any way because You assume JS'you can load the desired on the desired resolution, so the performance issue here at all difficult to identify. Although, of course, you can rely on this condition as "mobile is always slower than desktop devices," and of these considerations, if such a condition is "hard to turn" and raised to the rank of absolute - will also be logical to first do a "Mobile first".
Where and which sites are suitable (Mobile First) or (First Desktop) ;
MobileFirst, by definition, is perfect for those sites that initially (first) is calculated on the mobile device, for example some "music service online, with the ability to listen to MP3 shki as a 32-64Kbit, especially for those who have ran traffic and the Internet works with speed limits". The rest I described above :)