Now that all DLC packs are 400 MS points each I've decided to buy one, but which one is the better one?

I have Point Lookout and its absolutely brilliant. I haven't heard a single good thing about Mothership Zeta. Reviews seem to reflect this. 

Point Lookout is the best DLC of all the ones available for Fallout, well worth the price :)

Agreed - Point Lookout definately.  It is the most free roaming of the DLCs which works massively in its favour.

I enjoyed Mothership Zeta more than Point Lookout.

Point Lookout all the way my friend. I was very disappointed by Mothership Zeta.

Point Lookout, sadly I didn't get to enjoy it so much as I was too busy looking for achievements :(.
 
Definitely better than Mothership Zeta though.

Point Lookout. Mothership Zeta crashed on me half way through and every time I tried to load it would crash again. But I got the gist of it.

Dude, no question. Mother Ship Zeta is shit compared to Point Lookout. Zeta is super linear, and the only thing good about it is the alien epoxy (use it to repair your weapons) and maybe a few guns. Lookout on the otherhand, that one kicks ass. I won't say anything to spoil it other than it won't leave you disappointed.

Fun fact: the offer does not apply to the games for Windows Live service. Yes, I'm quite disappointed.

definitely Point Lookout. The only one that competes with it is Broken Steel, because of the level cap and the ability to continue your game after the storyline ends. The extended storyline is done very well too.

I was faced with pretty much this same dilemma. I only had Anchorage and Broken Steel, and I had little interest in The Pitt. 
 
So it was down to Lookout and Zeta. The solution?
 
Verbatim: "Aw, fuck it. They're 400 points each. I'll just buy them all, The Pitt included."
 
I think I made the right choice.

Point Lookout is easily the best Fallout3 dlc, buy mothership zeta if you want some repairing done or if you want a samurai armor and sword

Point lookout, gotta love the hatchet!  
 
 
KABOOOMMMM!

" Point lookout, gotta love the hatchet!    KABOOOMMMM! "

Point Lookout is probably the best Fallout 3 DLC, although Broken Steel did raise the level cap, haven't played Mothership Zeta yet, but from what I've heard it doesn't really compare to Point Lookout

Point Lookout was better.  But you could get them both for a paltry ten dollars....

BOTH