I was actually surprised that the first Gears didn't have a horde mode, given that Epic Games also made the Unreal Games, and Unreal had a gametype like that called "Invasion". I was even more surprised that Epic took Invasion out for UT3, thus guarenteeing that I won't bother buying it for a long time, if ever.
From what I've seen so far Firefight and Horde are going to feel very different, even once you account for the obvious differences between the way the two games play. Firefight the waves you fight are going to be more random, where as in Gears the waves are preset; so if you are finishing Wave 9 in Horde everybody grabs a Grinder because you know Wave 10 starts off with a swarm of those Beast Rider guys.
Firefight also has map's designed for it, where as Gear's Horde and Unreal's Invasion re-used the DM multiplayer maps. I have a feeling I'm going to be wishing that ODST gave you that option as well, as it greatly increases the number of maps you can play on. The random wave composition means that Firefight will probably have more replay value though.