The Empress Bass Compressor will offer you a touch more control (separate attack/release controls), significantly better metering, and the ability to choose between a truly "transparent" tone, or something with more tone colouration (two different options of that in fact). It's also worth noting you have a side chain loop/input available on the Empress, which can be used for a variety of things, most notable you could input something like a kick drum for classic pop/dance music style sidechain compression pumping.
The Cali76 CB will give you continuous control over the compression Ratio (as opposed to the three way switch on the empress), and can reach higher compression ratios in comparison to the Empress (although 10:1 is more than enough for most needs). The Cali76 also has this particular tonal quality in the way it adds some colour to the sound, and a punchy attack characteristic in the upper midrange which you can't really get anywhere else (NB the empress gets something similar to this in it's right hand tone switch position, though not as percussive/punchy to my ear).
There is no bad choice to be made between these two. You will be happy with either choice and both will serve you well in whatever context.
I would lean slightly towards the Empress, purely due to it's metering and the tone+colour options - I appreciate having a couple of instant gratification tone magic options on hand (or tonal transparency if so desired).
However, the tonal coloration of the Cali76 can be very addictive, and if you are familiar with and desire that sound it's the only place to get it really.
The
Tube Preamp/DI Sound Samples thread actually gives a very good representation of the particular tonality of the Cali76 as you can hear how it impacts various differences DI/Preamp setups.
TL;DR - two amazing compressors, with some subtle and nuanced differences. Empress delivers more versatility, Cali76 embodies a particular 'sound'.
Buy either blind and you will be ecstatic.