What's the difference between Puerto Rico and San Juan?
Game Play Differences
While the games are thematically the same, and San Juan was designed as the &quot;card game version&quot; of Puerto Rico, the gameplay is actually quite different.
Role Selection

In Puerto Rico your choice of role has many implications. You need to consider how the role will affect your success, how it will affect the other players, and every bit as importantly, how your success and that of others will be affected if another player chooses it. You're commonly making a choice that, for example, hurts you the least, or hurts the leader the most, etc.
In San Juan role selection has minimal affect on others. You do want to choose the role that's best for you, but that's about as far as it goes.

Strategic Play

Puerto Rico strongly calls for selecting an overall strategy near the beginning of the game and sticking to it, very broadly the &quot;shipping strategy&quot; or the &quot;building strategy.&quot; There are clever things you can do within each strategy, tactical stuff that requires good decisions every turn, but if you don't decide on a strategy early or switch strategies mid-game you're very likely to lose.
San Juan is much more tactical, with very little in the way of strategic play. There are a few cards that call for trying to push the overall game in a certain direction, especially the game-end bonus cards, and some cards complement each other better than others, but it still hard to call it real strategy.

Newbie Forgiveness

Puerto Rico isn't kind to new players, but worse, new players can easily throw the game to a certain player without realizing they're doing it. Sitting to the left of the newbie is an easy way to dramatically increase your odds of winning; seen from the other players' perspective it means the newbie can give you the game.
San Juan isn't a game you're likely to win the first or second time you play if the others are experienced, but you're not going to &quot;give&quot; the game to anyone due to sub-optimal play, and you'll likely score within 20% of the winner no matter how poorly you play.

Seating Order

Seating order doesn't just affect the possibility of a new player giving the game away in Puerto Rico, it also has a strong affect on how easy it will be to achieve your goals. If after a few rounds you decide that your game strategy is to grow and ship as much tobacco as possible, if the player to your right starts growing tobacco you're screwed. The same holds true for the shipping/building strategy lines in general: if the person to your right is using the same broad strategy as you, you're going to have a much tougher time.
Due to San Juan's general lack of interactivity, you're much less likely to be affected by seating order. I've yet to hear a compelling case for it mattering.

Teaching the Game
San Juan is definitely easier to teach to new players, though I wouldn't say massively so. However, it is much easier to teach how to play well than is Puerto Rico, and so is more forgiving.
Is It Worth Buying?
I absolutely consider it to be worth buying. If it's like any game, it's really most like Race for the Galaxy (see below for inside scoop), but a much lighter version. It's much faster to play than Puerto Rico, and is actually quite fun with just two players, something that can't really be said of Puerto Rico. The two games are so dissimilar in play style and feel, you'll never mistake one for the other in any way, even if you played them back-to-back.

Just a short bit of gossip about San Juan and Race for the Galaxy: the story goes that when Alea Games saw the (comparatively) great success of Puerto Rico, they started hinting around that they were interested in a card game version. One submission was a game very much like Race for the Galaxy, though many years earlier. The Race-like game wasn't selected as the winner, but the eventual game, San Juan, turned out to have incredible similarities, including the cards-as-resources, cards-as-money, and cards-for-their-face-function game concepts. Whether any ideas were copied or not, in the end Race needed a fair bit of redesign to make it less like what San Juan turned out to be.