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The Deacon's feet are longer, more organic, don't have a spur and they stand on their toes.  The Nostromo and Hadley Aliens are plantigrade. Not sure I'd call that "very similar".

@S.M

I updated the OP so that its the toes (not feet) that have the odd similarity, which's what I meant to emphasize. 

Not sure what you're pointing too. They both appear to have four toes (ditto the Aliens in the second film), but beyond that I'm not seeing it. They both have different basic stances. Deacon's stand on their toes and Aliens stand flat footed.

@S.M

To clarify further:

"Both toes have this distinct curve upwards, the ball point is very prominent, and the little toe is pressed inwards."

Not seeing it, sorry.

The Deacon and the "Big Chap" Drone and the "Hadley's Hope" Warriors all share a plantigrade stance. While it is true that the Deacon is shown to stand on "tip-toe" in its brief moment on screen, the anatomical structure of its legs is Plantigrade with shorter Tarsals than those seen in Digitigrade animals, such as the Queen - If a person walks on tip-toe it doesn't make them Digitigrade.

In terms of the composition of the hands and the feet of the Xenomorph are virtually identical in Alien and Aliens, unfortunately with creature design and development having been undertaken by ADI for Alien 3, Alien Resurrection, AVP and AVP:R have become increasingly generic, lacking the originality of Giger's original design, instead looking little different to your typical low budget, B-movie horror monster.