Seems to be different. I've sent off two consoles within a few months of each other, first one was replaced with a brand new console (same specs, but very recent mfg date) whereas the other was just repaired.
The one that was replaced was back to my door in under a week, whereas the repaired one took two weeks.
Probably something to do with work loads, stock, repair centre or even just the technician who looks at it.
I was well chuffed with the replaced one, because I bought it locally from eBay for £12 and the guy gave me the receipt showing it was bought just over a year ago, didnt even have to bother doing the RROD fix.