would it be better quality (lets say color space or quantity of pixels) an HDV-on-tape image captured by HDMI Intensity to AspectHD, than by firewire to AspectHD? In my case from a Canon HV20.

you will get better quality using the intensity by bypassing the hdv compression that occurs going to tape

But if you're playing back your HDV tapes over HDMI there is zero quality improvement whatsoever. HDMI ingest only has an edge if you're capturing 'live'.

I belive you would however capture at 1920x1080 rather than 1440x1080 (blown up from 1440x1080 however). Yes....you can do this in post but I believe it is probably best to do this in cam (IMHO...I might be wrong).

Likely the opposite, software upscaling algorithms are more flexible than is probably in consumer hardware. This is not a big factor either way.