Drop Dead is more like"House of the Dead" in front facing mode, it's a polished game with scripted fights and a story, but it's cartoonish graphics are seriously outdated (think 2D textures with no bump mapping applied on low polygon models), and while fun to play through it's not scary at all.
Hordez is more like "Left 4 Dead" in 360° mode, it's not polished (zombies clipping through walls and getting stuck behind doors), doesn't have a story and is more about non stop shooting at mindless waves... but the graphics are creepy and on my first run it was a jumpfest.
So if anything, they're so different you should have no problem choosing between them : - Drop Dead if you have fond memories of "House of the Dead", in which aiming and reloading does matter. It doesn't even get difficult before reaching the second timeline (I heard the third timeline is really challenging). - HordeZ for a short but intense mindless session of "Left 4 Dead". It's more about "keep shooting" then about aiming, and on my first run (of the two first levels) I got jumpscared enough times to make me run out of jumpscares.