Ilford say (though explicitly don't guarentee) that the new RC's are believed to be as archival as fibre - though they honestly admit that only time will judge that accuratly.
I've used mainly fibre. However as I'm working in a "home darkroom" (ie the sort with a toilet and shower in it) my proceedures aren't as rigiouros as perhaps they should be. Under these conditions I suspect that RC may well fair better: Fibre processed well may be better than RC handled correcly, but as its more porous fibre is more vulnerable to poor washing, or fixing. I suspect that poorly processed RC will last longer than poorly processed fibre.
Of course the correct answer is to be more carefull and process more carefully...