Best game for two couples, median age 20?
No one can really answer your question directly, but I can give an overview of the four different games and their play styles.

Agricola


Complex rules and might be a bit heavy for a non-gamer initially.
About building a farm.  Everyone likes building a farm.
"Aggressive" play comes from resource denial
Most games will take 30 minutes per player to play.  During learning it'll probably take an extra 15+ minutes per player.
Lots of setup/packup work is required.
Plays best with 4 players.
Highly replayable and highly addictive.


Dominion


Rules are relatively simple.
Very little aggressive play.
Player interaction with each other is limited.
The game plays as a deck building game.
Most games will come in under half an hour.  Some cards will extend this but never more than an hour.  
Plays best with 2 players.
There are a lot of worthless cards/card combos, which limits the combinations of possible games more than it seems initially.
Lots of replayability, but expansions are highly recommended to open up the possibilities.


Puerto Rico


Rules are moderately difficult - easier than Agricola, more difficult than Dominion.
Play is generally not aggressive, but has a high degree of player interaction.
Game time is rarely more than 1.5h.
Plays best with 4 players.
Moderate amount of setup/packup work.
Base game is slightly imbalanced, favouring players who start with corn.  This won't have too much of an impact though.


Settlers of Catan


Famous mainly as a "gateway game" into Euro-style games.
Lots more luck involved than the other three games.
Some games will last over 2 hours, depending on luck factors and how decisive each player is.
Plays best with 3-4 players.
Rules are relatively simple to learn.
Dice use will make new players feel more comfortable.
Pretty quick setup/packup time.
Probably has the least long term replayability.  Certainly without the expansions this one will not amuse as long as any of the other games.


My recommendation is actually Puerto Rico, based on what you have described.  I would then recommend Agricola after you have played Puerto Rico a few times and have mastered the mechanics.  I still regularly play both games despite having played them hundreds of times.  I very rarely play settlers and occasionally play dominion.  Dominion's a game I play to fill in half an hour to an hour of time. YMMV.

Certainly if you do not wish to buy expansions the best choices are Puerto Rico or Agricola.  Settlers becomes a lot more fun with the Seafarers expansion and Dominion really opens up as soon as you get at least one expansion.