Hi, I started the modular three month ago and I first bought the PR because I think it was the most beautiful set but it was not the best idea.
When I saw that the PC and PS were out of stock on the Lego Shop, I run to my Lego Store in Disneyland Paris to buy both the sets.
If you don't have unlimited money, if I were you I will buy the PS because of the fear to be retired soon (but according to a guy on an other topic on this board, it should be retired late this year - http://www.eurobricks.com/forum/index.php?showtopic=107549entry2184929).
So if you want all the modulars currently available, I will buy the set in this order: PS, PC, PR, DO