If your looking strickly for content, I personally would say go for R/C Car. Both R/C Car and Xtreme have excellent content, but the Xtreme people need to grow up a little with the slapping of womens pictures all over the mag. Its a little pointless to me.
On the other hand, if your a newbie, RCCA(Radio Control Car Action) might be a better choice. Their always running articles for people newer to the hobby. Many of us here complain about the ads in RCCA, but they do serve a purpose by letting you see whats new and being able to easily compare prices between mail-order shops.
The best thing you can do is pick up an issue of each at the news stand and see which you like best. Then subscribe to that one if you wish. Sounds like a cop-out answer, but I do think its the best way to do it.
Remember, just because you subscribe to one mag, doesn't mean you can't occasionally pick up one of the others if they have an article or two that you want to read!