as it says on those github pages, one is a zdoom fork while the other is a gzdoom fork. of course brutal doom needs the GL features to work properly. i actually got the regular release of v21 to run acceptably with this version, the only problem i've ran into is that upon a midi track ending there's a message complaining about lack of memory despite having another 250mb of free RAM, and i had to restart the audio system to get the music back.

one useful tweak for anyone running into this issue: i've been wondering why this fork ignores the refresh rate set in windows and just runs at 60hz (which is kept even after exiting the game, at least under 98se), but as it turns out, there's an ini variable "vid_refreshrate" for this and its set to 0 by default.

It's the same but one has the GL renderer and it's in a branch inside the gzdoom repo. I recently moved ZDoom32 to another repo but i dropped ZDoom LE. The refresh rate works the same as in ZDoom.
https://github.com/drfrag666/gzdoom/commits/gzdoomle

ZDoom_LE_2.8.3a working well on my recently upgraded Pentium3 933mhz with MX440. Completely new to source ports apart from FastDoom but this gives me lots of advantages for this faster vintage PC. Also able to open One Humanity and Sigil WADs, also Brutal Doom mod v21 working fine. Getting framerates of 140 at 640x480. Thanks for this

I can't switch to OpenGL because my G450DVI seems to support 1.1.3. only and not 1.2.
So hires-textures are not possible, but brutal-doom works good in software-mode.