Any of these two series is fine to start with, not that it really makes any difference. There are a lot, and I mean
a lot, of running gags throughout all the books, and a lot of recurring characters, which is why he gets funnier the more you read him - that's the only reason to start at the beginning. The Antipope, first of the Brentford trilogy (which is definitely
not three books) is the first written, and you'll get to meet Pooley and O'Malley in exotic Brentford