I use both mixers. They are excellent (and no noise problems on the StMix!).
My most frequent configuration: StMix and/or A-138s as sub-mixers (either of which can end up on a stereo effect: reverb, Data Bender, etc.), and the Black Stereo Mixer V3 as the final mixer before the Listen IO output module.
Some details differ (see the manuals) like a 5th input on the Befaco or the merge of mono channels on the Erica Synths.
The choice will depend on the context of use.
'On ne devrait jamais quitter Montauban' (Fernand Naudin).