Strange, their integrated magnitudes are similar (NGC 5128 a touch brighter) but NGC 5128 is a much larger object and its surface brightness always seems considerably less than M104 to me.
Might be a 12" thing!
Observing through a little scope, M104 is a bright little gash at low powers, but the dust lane can be seen. NGC 5128 is seen as a much bigger dull nebulous object that looks like a fuzzy pacman on a bad night but on a good night it is larger again and the dark lane can be seen right through. M104 holds really well at higher powers making the dust lane clearer while NGC 5128 is not worth viewing at higher powers, yuk!
Just my experience, they're like chalk and cheese to me.
Cheers -