I've fired the Deagle .50, and was not impresssed. It's a big, expensive, clunky toy, a perfect example of a "Hey, look at me!" gun. Favorite of moviegoers and the uninitiated.
I have not fired the S&W .500. If I lived somewhere where there were really big bears, I might want one. I don't, so...I don't. In my "real world", a .357 or .41 Mag is enough for anything I will ever do. If I move to AK/MT/WY ( a possibilty, but a remote one), I will re-evaluate.
If I owned either, I would handload, because ammo for either is prohibitively expensive (if you want to practice enough to attain a modicum of proficiency).
I like the .50GI...it is fired from a 1911 or Glock frame, both of which are concealable firearms. The ammo is expensive, but is also reloadable. This is the only .50 that has any attraction to me.
Of course, I don't own anything I cannot carry or shoot somewhat regularly (preferably both).