I won't chime in personally on preference, cause they
are very different / have different purposes as pedals.

But on the differences...
They're both high-gain on full, but in different ways (and with pretty different results!).
The Algal Bloom has dirt+gain+fuzz, resulting in the final sound. Then all of the controls that act on that.
The AB-Synth has fuzz+gain+fuzz, (layered fuzz), then the Trip mode.
The Algal Bloom is about natural-as-possible fuzz, fantastic note separation, amp-like, ideal fuzz. Cranked, it gets a little scambled edge, and dialed back has lots of nice overdrives. Like a great classic fuzz with some upgrades and a wider range.
The AB-Synth is high gain all-around without much gain reduction (but a nice texture control and fuzz/shape control). It's surging,sizzling, juicy high gain fuzz!

Like the AB, it is very harmonic in nature (more overtone here). Not your traditional oscillating fuzz (long sustain in oscillation mode that then turns into stutters as you sustain...then blips). Roll back on your volume control for octave down and synth bass.