I have played both extensively, so I would argue that both are excellent games, and vary in what they achieve:
Puerto Rico takes longer to setup and also a bit longer to play with same players compared to Race.
Race has been developed specifically as a card game variant of Puerto Rico.
It therefore squeezes all the good elements of Puerto Rico into a cardgame.
Both games are unique, though you can tell the roots of Race when you know PR.
The difference is:
Since Race is a card game, the luck factor is dependend somewhat on the cards you draw, although Race does a very good job at minimizing this as much as possible by giving the cards multiple usage also as money. Puerto Rico is the more hardcore of the two, although there is a factor in PR that has been elegantly eliminated by Race: You can get lucky if you sit next to a (new) player, that does suboptimal moves which help you get an advantage!
This is eliminated in Race with a hidden, simultanously role choosing mechanism and the fact that players don't interfere with the other players cards (at least in the basic set, which is really all good by itself and doesn't imho need any of the expansion cardsets).
So I would guess Race is a safe bet to get into the game mechanism and Puerto Rico should be checked out if you want to master the very same game mechanism with a group of dedicated players.
There is also a very good tutorial video for Race, so this also would speak for Race as first choice for testing it out:
http://www.weihwa.com/~whuang/videos/rftg/rftg.htmlConclusion: Both are excellent games that would appeal to anyone liking Mule or resource related games in general.
