"Although the Blu-ray Disc specification has been finalized, engineers continue working to advance the technology. Quad-layer (100 GB) discs have been demonstrated. And TDK announced in August 2006 that they have created a working experimental Blu-ray Disc capable of holding 200 GB of data on a single side, using six 33 GB data layers."
HD-DVD can add as many layers as possible, but it will never be able to hold more than Blu-ray which can add additional layers as well.
True, HD-DVD can hold more by adding more layers, but so can Blu-ray. HD-DVD holds in 3 layers what Blu-ray can hold in 2.