Depends. Clang has some advantages (compiles faster, is much easier to extend, license). But so does gcc (works on anything, license). Which produces better code depends, I've seen huge differences between the two for both sides (usually, but not always, icc > gcc > clang). Very long-term clang has the advantage since their code is less fucked because it's more recent.
>>817532
>if I use even a small fragment of it in my private project I will be forced to release all my code
You only need to release the code to your users. Since the project is private you have few users and can add in the contract they can't develop a competing software.
>>817497
You don't always need maximum speed. Often you only need "does x before y seconds pass". Take the guidance software for a rocket for instance. You can afford wasting a few more watts to avoid some crazy bug.