Here's a benchmark comparing pi-hole running on a Pi Zero (with USB ethernet) vs a Pi 3b+.
tl;dr There was negligible performance difference for blocked domains, but a measurable difference in mean for forwarded + cacheable domains. Although the Pi 3b+ has a 11ms better mean response time for forwarded queries, the P95 for pi zero is better in both blocked and forwarded queries.
I would recommend using the Pi Zero.
Prior to the experiment, I assumed that the pi zero would be 30-50% slower in all cases, and that stddev would be larger (worse & more erratic latencies).