About four letters and a period.  Actually, the difference is more a matter
of attitude than anything else.  Both groups tend to deal with similar
subjects. However, in AR, you *are* likely to get flamed for having a
problem with a relationship.  ARC is much more friendly, but does drift off
of the strictly 'romance' topic from time to time, with stories and poems
and general silliness (that's why it was created in the first place, BTW).
There are people who read both groups, and enjoy both.  Others seem to
think that one group is better than the other. *shrug* It takes all kinds.

We should warn you though: cross-posting between AR and ARC is generally
frowned-upon by some folks in AR.  They seem to think we're inflicting
ourselves on them.  Go figure.