They are very close in size, as they are both intended for use in Secrets (and I personally use SOTR zombies in Incursion when the hordes get big!). The visual style is a little different, since the Incursion sculpts were sculpts commissioned from well-known sculptors by Grindhouse while the Secrets sculpts are West Wind in-house sculpts. In the fluff and rules, the Incursion zombies ("Sturmzombies") are a little different from the "normal" German zombies represented by the WW sculpts, which is also why they look a little different.
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john bailey