[MatrixDweller]

I'm pretty sure that both of these amps have toroidal transformers. Copper is a better conductor of heat than aluminum so a smaller copper heatsink would be more efficient than a larger aluminum one. Excess weight can also mean poor design or heavy use of older (heavier) components or materials.

I think it would boil down to features, which the Onkyo has more. You get network capabilities (streaming, internet radio), THX Ultra certification as opposed to THX Select, a little more wattage and much better video upconversion.

The 905 has the upper hand in pretty much every category, looking at the specs. You might want to audition them though to see which one sounds better in you opinion. On paper the 905 is a clear winner.