I have Black Hawk Down and the single player part is pretty bad at times. The AI for your teammates as well as the enemies is horrible with guys running into walls as well as pushing you out of the way. In some levels it plays a lot like an arcade shooter by just manning a gun on a Humvee or a Black Hawk. But the multiplayer element is pretty fun.
I've only played the Raven Shield demos and from what I played it seems like solid game.
I'd say Raven Shield, having played both I enjoy Raven Shield much more. BHD just wasn't my cup of tea, and while the graphics look nice in screenshots, in-game there's just something "off" about them (esp. the human models) that I don't like, and the gameplay just isn't as good as RS. BHD just feels a lot clunkier and less refined. Both have demo's out though, so if you really want to see which you'd prefer, I'd suggest downloading them and giving them both a try before buying .
My friend has both so I went over to his house to play em. BHD is a solid game but there is much more variety in Raven Shield. And like Ryan said the graphics are smoother and more refined. So I picked up RS for $40 at Circuit City and am currently getting spanked in multiplayer, thanks guys.