The answers so far have been good, but I think it's hard to get the picture in such a brief context.
In terms of control, Anniversary and Series I electronics are the same (if you upgrade the Anniversary Q-switch to a 3-way). Series II adds a master volume and the continuously variable Q controls. These three setups are all stereo -capable where you can route each pickup through a unique signal chain to their own amplifiers or recording channels.
The important differences, in my mind, are as follows:
1. Pickups. The Series packages use wide-aperture single coil pickups with a dummy hum canceller located a few inches from the actual pickup. There is a set of internal pots to adjust the balance between the pickups and the hum canceller for maximum noise reduction. The Anniversary package comes with regular-aperture pickups. The hum cancellers for these pickups are built into the housing. An upgrade to FatBoy wide-aperture version is possible.
As previously mentioned, the single coil pickups are described as having a more open sound. I would say they are more articulate in the upper frequency ranges that complete the character of the tone.
2. Electronics.
Series electronics are powered through the 5-pin cable. While the controls are similar to the Anniversary package, the guts behind them are not the same. The additional headroom in the power system for the Series package should translate into lower noise from the onboard preamp. Not that the Anniversary package is noisy, but that the Series package is as quiet as rack-mount gear. You can run Series electronics on batteries, but they eat them up very quickly. Non-Series electronics run relatively low power. A single 9v will give you months of service. I wish we could run the Anniversary circuit at 18v, but Alembic hasn't offerred that as an option. Also, the CVQ (continuously variable Q) controls are only available as part of the Series II package. If you want that level of control, you have to go to the top.
There are differences in standard options between these instruments, but they can all be added or removed between the models. Things like plated hardware, LEDs, and wood recipes can all be upgraded on the lesser models.
In the end, it's up to your ears and your needs. Your profile says that you own a Spyder. As such, you know what Anniversary electronics sound like. Do you ever use them in stereo? If not then you might not really appreciate the subtle differences between Anniversary and Series electronics. I believe the Spyders came with AXY pickups, you might try the relatively inexpensive path of ordering a FatBoy pickup and installing it in your Spyder to see how you feel about the wide-aperture tone. This should get you closer to the Series sound. Either that, or get to a dealer with a Series bass in stock where you can hear it for yourself.
