can't speak to the differences, but two cool choices.
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IIRC, the ET will be a much more forward sounding speaker than the Spica. The Spica is a neutral, revealing design that is limited in bandwith at both ends, with highly damped bass that's on the "dry" side.
The ET is more dynamic with real sparkle in the treble and a bit of underdamped mid bass hump.
Ironically, the dynamic cone design Spica might, in a way, recall the old Quad panel sound, while the panel design ET feels more like a dynamic speaker (albeit with an unusually appealling highend).
Qualifier: This characterization is from looong ago memory that might have been damaged by the passing of time.
So take it FWIW.