  Spatial linear transforms that process multiple parallel analog signals to
simplify downstream signal processing find widespread use in multi-antenna
communication systems, machine learning inference, data compression, audio and
ultrasound applications, among many others. In the past, a wide range of
mixed-signal as well as digital spatial transform circuits have been
proposed---it is, however, a longstanding question whether analog or digital
transforms are superior in terms of throughput, power, and area. In this paper,
we focus on Hadamard transforms and perform a systematic comparison of
state-of-the-art analog and digital circuits implementing spatial transforms in
the same 65\,nm CMOS technology. We analyze the trade-offs between throughput,
power, and area, and we identify regimes in which mixed-signal or digital
Hadamard transforms are preferable. Our comparison reveals that (i) there is no
clear winner and (ii) analog-to-digital conversion is often dominating area and
energy efficiency---and not the spatial transform.
