It's a toss-up, Andy. Each model has 6 controls; the Europa electronics are more versatile in that they have the quick tone change switches... the bass and treble boost/cut switches that can boost or cut 6 dbs...
The Signature electronics lack these toggles, and instead have separate filters and Qs for each pickup. Since this allows separate tone modification for each pickup, the Signature electronics are more versatile in that respect; but the Europa electronics are more versatile in that respect -that the bass and treble can be adjusted. (Both models have a master volume, balance and 0/+8 db Q switches).
My advice: choose the best of both worlds and get both. Get the Signature electronics and ALSO add the bass and treble boost/cut switches TOO. And I'd recommend paying the extra for 3 position Q switches (0/6/9 or 0/4/8 or something else).
I don't believe the addition of the quick change switches -nor the upgrade to 3 position Q switches- are terrible pricey; and the versatility they add is tremendous.
I know because my custom Europa (COTH, June 99 8 strings of power -in the custom archives near the very bottom) has 3 position Qs AND bass and treble -and mid- controls for each pickup... and they work VERY well.
You could get a really simple and awesome control configuration going... 4 knobs -volume, balance, and two filters forms a diamond shape... and between each of the knobs, you'd have 2 toggles (8 total): separate bass, mid, treble, and Q switches for each pickup.
Personally, I think my Europa sounds better than any Alembic I've ever heard, even the series... but then it also has a LOT of ebony in the neck, and I've never heard a series bass with ebony.
Anyway, good luck. I suggest considering this COMBINATION/blend of Europa and Signature electronics... the best of both worlds.
I can tell you from experience, it's worth the price (at least to me).
The tonal versatility of my bass -with 3 filter/Q controls AND 3 eq controls FOR EACH pickup- is in a class by itself.
My interpretation of the price quotes is that these options aren't terribly pricey; mid-range switches probably would be the most pricey, but they did 'em for me, so they should still have the specs.
By the way, the mid-controls are EXTREMELY useful.