...hmn...
On the one hand, Iron Will probably makes them resilient to chaos as well as more effective against the orks.
On the other hand, natural weapons could be something like venomous thorns, making them more broadly-useful assassins and ambushers.
Ultimately, I think Iron Will is better. Probably less likely to be used against us, while still being very helpful.
Iron will is likely to be the most useful, as natural weapons aren’t especially effective against a species best dealt with from the maximum range of the nearest artillery piece. Similarly, the ‘Nids will likely simply adapt, or at worst copy, any natural weapons our ‘auxiliaries’ have
Iron-Willed
+ 1 to iron will
The Xenos are
Iron-Willed, rejecting the needs of their body and the weakness of the mind alike. Admirable, if applied to the correct ends.
7
The Xenos are afflicted with
Slow Reproduction relative to humanity. After all, it takes time for the womb-sprouts to grow from soil that meets its needs and release their young, which must then go through a lengthy maturation cycle before it, too, can release spires.
3
The Xenos are
Delicate in the sense that they are exactly as frail as they appear to be. Perhaps one-fourth the size of a human, they lack the resilient constitution Terra has given the Emperor's chosen species and must content themselves with their soft, squishy frames. Of course, most threats to mankind are dangerous enough it makes little difference that they're frail next to humans, but it doesn't do their staying power any good.