I am going to get the LEgendary Edition on PC. Is there any reason to choose the Steam version or the disc version?

There are no reasons really; it's as you wish as anyway you'll need to activate the game in Steam. The files on the disks come without the executables; these are installed only by Steam.

Sadly this^....welcome to the future.

I'd say price is the main reason. For quite a while, Skyrim PC DVD was a lot cheaper on Amazon than it was on Steam, as was Dawnguard (which was only an activation code, no disc).