If we’re talking about an outside receiver, then Short in Elite is the obvious choice since slot o matic only works in the slot.
If putting this ability on a slot receiver, and assuming AP cost is the same, then I would go with Slot O matic. Both abilities give you better cuts on routes but slot o matic adds boosted catching inside 10 yards from LOS, both inside and outside numbers, where short in elite only boosts catching within 10 yards of LOS inside numbers.
TLDR: Slot O Matic is better ability with more benefit, but only if your receiver is in the slot. If you use them outside or move them around, then you’re better off with short in elite.
Which one?