I've actually been going through this same bit of thought over the past few months.  I've found that popularity seems to be split right down the middle between Cruisin and Rush.  I have a slight preference to Rush, but enjoy both and would probably be happy with whichever is available for less.  It's very common to hear that Rushes are available for $600, but in my area, I haven't found a single Rush for sale at all, let alone at that price point.  I can locate Cruisin USAs anytime from $700 to 750.  
As a wildcard in all of this, I can also have a Virtua Racing Twin *delivered* for less than what a single Cruisin or Rush unit will cost.  I'm a fan of VR as well, which makes it a good option; however, it lacks the variety that I believe Cruisin USA and Rush both have.  Team that with the upgradability of both Cruisin and Rush, and Virtua Racing becomes a tough sell, despite the low cost.