For defense INF are the better buy.
3 IPC purchases 2 points and a unit of defense if you do INF.
5 IPC purchases 3 points and a unit of defense if you do ARM.
15 IPC is 5 units at 2 points each if you buy INF.
15 IPC is 3 units at 3 points each if you buy ARM.
A quick and dirty way to determine value is:
attack or defense value (depending on which way you want to use the unit) + the number of units.
So on defense…
15 IPC as INF is
5x2+5=15
15 IPC as ARM is
3x3+3=12
15 IPC spent on INF is 25% better on defense than the same IPC value of ARM.
And that difference is multiplied if there are FIGs backing up the INF so that the INF shield allows the FIGs to fire more rounds of battle before being taken as losses.
The kicker is to add SKEW to the defensive posture… or as I refer to it… ANCHORS.
You need the INF for bulk… to absorb attacker hits, and to create a few hits against the attacker. But you need anchors, preferably FIGs that will most likely score a hit every round of combat, to chew into the enemy forces while your INF shield the major defenders from enemy attack.
An large INF stack with a several FIGs is the best defense you can get. In that situation, ARM are a just a bonus, and ART are no improvement over INF.