If it was a choice of the two, I'd say that it depends on what you're looking for. If you want a lengthy one, Point lookout is for you. It took me so long mainly because of how many locations there were to find, a good variety of quests, and some challenging enemies littered around.
Zeta has the same features, but the constraints on it is that it's not too long, or demanding as far as quests go, and as Chriz stated, a lot of the ship is inaccessible after you finish the main objective making it pointless to be able to complete things like finding recordings. All you get is the bridge, engine core, and the initial security area you started in. Not to mention random encounters with a massive amount of aliens and their support drones out of nowhere thanks to either a glitch or respawn.
As fun as Zeta is, I'd go for Point lookout simply because it had more to offer than Zeta did.
