I only have the NECA releases - they're ok in terms of articulation, but nothing exciting.
They're good value for money if you simply want a display piece which has nice detailing that has a bit of articulation (see my pics
here).
Revoltech also makes an Alien figure, but you can pretty much get two NECA ones for the same money.
I guess it depends really on how much articulation and detail you're looking for and how much your budget is.