The VPL-HW15 replaces the HW10, and in doing so, offers up an improved, but every similar projector, for less money. We'll call that evolutionary, rather than revolutionary. While the MAP pricing on the older Sony was $3499, the VPL-HW15 comes in $500 lower at $2999.
For the most part, the Sony VPL-HW15 home theater projector looks, and cooks, exactly like the HW10. Brightness is about the same. They look virtually identical (I confess - I haven't taken photos of the HW15, so the product shots you are seeing are actually the HW10).