Hi Eduardo
For a similar design there is a big difference in performance between SCM19's and active SCM20's. Otherwise i feel the difference is subtle. However to answer your other question (SCM40's thread earlier on today) all things being equal upgrading from SCM19's to SCM40's gives significantly greater quality in areas such as LF and particularly (due to ATC's state of the art studio Soft Dome drive unit) mid range performance
http://www.atcloudspeakers.co.uk/studioDriveUnits.php
Fwiw, i feel that SCM40 monitors are probably the finest VFM passive speakers currently available on the market regardless of price.
ATC monitoring. The sound of accuracy.
All the best
Rick @ Musicraft