by Leonithic
I've played both, and bought San Juan.RftG clearly has more depth to it, but this depth can come at the expense of a lot of initial confusion.
I can explain San Juan to anyone in five minutes and be off and playing.
Also, for reasons perhaps alluded to earlier, San Juan is just more social. I always have a lot more banter going on during San Juan than any RftG games. (And RftG banter around here tends to be like, "Wow, good choice of discarding your Scintillating Potato on the Alien Ramen Factory to chain into your Wobbling Rebels for three VP and nabbing the Most Sodium-Filled award." Not chit chat.)
Like others have said, it depends what you want, but for me San Juan fills a great niche, just light enough with some thought required to what you need to do. Sometimes I want something a bit meatier than Fluxx or Bang! to finish up a hot night of Goa or Lords of Waterdeep.