They are very different... The Arcane sounds like what you'd expect a great early to mid 60's P bass pickup to sound. Clear, but slightly aggressive mids with an airy top end that sits great in a mix. I find it interesting that many look for a pickup that is warm and thick on its own when the truth is, that sound turns to mud in a mix. Slightly more aggressive mids and a little bit of brightness sounds warm and thick in a mix

Anyway, I am a huge fan of the Arcane and I'd recommend it to anyone looking for a great vintage voiced pickup.
The Harris(SPB-4) sounds more like a late 60's Fender pickup to me... stronger bass, but with a bright top end. Its difficult to describe in words, but this pickup hits hard down low, and sounds GREAT in a busy mix. Its got a clear supportive sound that fills out low end without getting in the way of guitars/keys/vox etc. I'd say one of the really great sounding pickups for a busy mix when you need to cut through a bit more.