I think in the article I say that Feel No Pain is great for any unit, but I wanted to stress that a big unit sitting in cover with Feel No Pain will be unreasonably hard to dislodge.

Ultimately which unit you give Feel No Pain is up to you - what unit do you think needs it more, i.e. which unit do you expect to be taking more firepower? Typically that would be a close-combat unit, so I would say your assault unit. However, be aware that some armies will prioritize your ranged unit because of the damage they can do to certain units - i.e. Tyranids, Daemons and Orks will probably target the ranged unit more with their own long-range weapons. In that case, the points may be better spent on them.

There's also the valid 'point-per-model' cost of certain upgrades that affect an entire squad, and a good example would be Psybolt Ammunition for Grey Knights. It is the cost of a Strike Squad member to give all Storm Bolters in the unit S5. Now, the value of Psybolt increases exponentially the bigger the squad; a 5-man squad means you are essentially paying 4-points-per-model for it. A 10-man squad, however, pays only 2-points-per-model for the upgrade. This may seem trivial but it is a good way to work out when an upgrade is worth it for a unit if you can't decide and want to resort to number-crunching. As such, bigger squads would benefit more from expensive upgrades like the Icon of Excess.

Of course though, if the upgrade is worth it, then you are likely not to care about that ratio anyway.