3 str is actually 6 AP and it's almost 0,5 DPS.
But anyways, say you are a tauren warrior with a shitty one-hander (14-28 damage, 7,8 DPS and 2,7 speed) and a shield in your 20s, you have about
:
60 str = 120 AP (+40 for being level 20, +35 from battle shout) = 195 AP. Which brings us to: 51,6 min damage - 65,6 max damage and 21,7 DPS.
65 stam = about 650 health?
30 spirit = 30 health per 2 seconds out of combat (30*1,33 +23 = 62,9 while eating food)
Time to recover 30% health after killing a mob: 214,5 health will regen over 7,1 out of combat, 5,3 sitting, 3,4 while eating
now when picking a new statstick with either +3str or +3spirit you'll have next upgrades:
+3 str =52,8 min - 66,8 max 22,1 DPS
+3 spirit = 32,4 ooc; 43,1 sitting; 66,1 while eating.
So implying you lose 30% health per mob, you'll have to regen 214,5 health, it'll take 6,6 out of combat, 4,9 sitting or 3,2 eating.
So there is that, difference is minuscule 2,4 health per tick (2 seconds) is really nothing, but keep in mind that spirit ooc health regen grows linearly and gets less effective the more of spirit you stack (due to health regen formula being "spirit*0,8+const") the higher level you are (because it can't keep up with HP growth), and all it achieves - reduced downtime between mobs, it doesn't provide enough of a bonus to trump eating (or bandaging) completely (time it takes to regen by simply standing/running around is twice as long as eating food of appropriate level), but reduces your DPS (which makes you spend more time IN combat, taking damage and not regening health). While your str gain also affects AP linearly (AP=str*2+(level*3-20)), so stacking more of it doesn't slow down at any point and DPS is always useful for leveling, as it reduces amount of time you spend in combat (you lose less HP and get XP faster).
I have excel spreadsheet open right now but can't be bothered uploading screenshots, if you wish to see comparison (how it affects H regen and AP) between "spirit gear set" and... regular gear. Specify level, race, and your definition of "spirit gear set" and i'll do it in free time