It's probably too late to get back on the topic of putting some numbers to the religious vs. humanist idea comparison, but let's take a stab at it anyway; I'm trying to figure out which one to choose from in my current game anyway.
We'll have to distill each idea group down to the core ideas and bonuses that tend to have the broadest impact in game first. This will necessarily disregard any synergies with particular nations or religions, but we just want to get a baseline for comparison at this point.
Religious
- Religious CB
- +1 missionaries
- -25% stability cost
- +3% missionary strength
- +1 tolerance of the true faith
+1 yearly prestige (prestige is too easy to farm in war)
+2% missionary strength vs heretics (heathens generally outnumber heretics in the world)
-25% cultural conversion cost (waste of diplomacy points no matter how you slice it)
Humanist
- +25% religious unity
- -2 unrest
+3 tolerance of heretics (heathens generally outnumber heretics in the world)
- -10 years of nationalism
-50% cultural threshold (loses effectiveness as you grow)
- +33% better relations over time
- +3 tolerance of heathens
- -10% idea cost
Most of the remaining ideas above are related to unrest management in one form or another, so let's consider a specific scenario: conquering a heathen province with an unaccepted culture, raising autonomy once, coring it, and then converting it. The basis of comparison will be made in terms of unrest-months. We'll assume that both idea groups have been maxed for simplicity, and some reasonable assumptions will be made about the typical status of a nation.
Unrest modifiers for newly conquered wrong culture heathen province:
- War exhaustion: 0 (often negligible, and can be reduced by Innovative, Defender of Faith, or diplomatic points)
- Overextension: 3 (equivalent to 60% overextension, because you fed half your conquest to vassals)
- Legitimacy: -1 (from +100 legitimacy)
- Stability: -1 (from +2 stability)
- Religious Unity: 0 (too small to matter for religious idea group, and non-existent if humanist)
- Nationalism: +15 for religious, +10 for humanist
- Unaccepted culture: +2 for religious, 0 for humanist (let's categorize the humanist global -2 here since we will never convert cultures)
- Heathen tolerance: +3 for religious, -0.5 for humanist (using roughly 2.5 intolerance as base)
- Raised autonomy: -10
The total unrest to begin with is therefore:
- Religious: +11 unrest
- Humanist: +0.5 unrest
With 36 months of coring time, and nationalism ticking down by 0.5 per year, this period will generate:
- Religious: +378 unrest-months
- Humanist: +6 unrest-months
Now we send in the missionaries (+6 unrest) after 3 years (-1.5 nationalism, -3 overextension), and the unrest will be
- Religious: +12.5 unrest
- Humanist: +2 unrest
If religious can convert in 12 months and humanist in 24 months, because we have to make up the numbers as we go and we might as well pick nice ones, then we get:
- Religious: +150 unrest-months
- Humanist: +48 unrest-months
The final unrest modifier, with increased autonomy still active, is then
- Religious: -1.5 unrest (-1 legitimacy, -1 stability, +13 nationalism, +2 culture, -4.5 true faith, -10 autonomy)
- Humanist: -8 unrest (-1 legitimacy, -1 stability, +7.5 nationalism, 0 culture, -3.5 true faith, -10 autonomy)
Thus in terms of just counting unrest-months, humanist probably has a sufficiently large advantage to give you a choice of either trivializing unrest, or not raising autonomy to begin with and squeezing an extra 25% out of newly conquered provinces. The extra missionary from religious ideas will allow more provinces to be converted at the same time, but that is somewhat balanced by the extra unrest-months that it will generate. Furthermore, conversion is only necessary for humanist so autonomy can be lowered sooner with tolerance of the true faith in effect, so the slower conversion rate of humanist is not a significant detriment in this regard.
Now lets take a second look at the two idea groups, crossing out the ones we just used for comparison:
Religious
- Religious CB
+1 missionaries
- -25% stability cost
+3% missionary strength
+1 tolerance of the true faith
+1 yearly prestige
+2% missionary strength vs heretics
-25% cultural conversion cost
Humanist
+25% religious unity
-2 unrest
+3 tolerance of heretics
-10 years of nationalism
-50% cultural threshold
- +33% better relations over time
+3 tolerance of heathens
- -10% idea cost
This looks a lot more manageable now, and the remaining ideas appear to give religious ideas a slight edge in terms of monarch points: the savings in monarch points from the religious CB and reduced stability cost are very likely to outweigh the savings from idea costs. If humanist is the first pick and maxed out, it will only save 1960 monarch points over the whole game, provided that all ideas are taken without any refunds. Assuming very conservatively that you can reach religious ideas 3 by 1500, and only have until 1650 to use it, the two ideas would break even once the religious CB and reduced stability cost can be translated into a savings of 13 monarch points per year. That is the equivalent of making one unjustified demand in provinces every two years, and seems like a very reachable target.
The only left over idea now is better relations over time. This is a pretty powerful idea, although its impact has been reduced somewhat in the current version of the game since coalitions dissolve after war, and AI nations cannot join coalitions against an aggressor while a truce is in effect.
In summary, religious and humanist ideas are pretty close to each other under this simple analysis. Humanist is better for unrest management and maintaining good relations, while religious will save you more monarch points if you can use its CB frequently enough. The tie-breaker will probably end up being your choice of other idea groups. For example, a nation that is already maxing out exploration and expansion for the colonial conquest and overseas expansion CB may not need the religious CB for enough provinces to justify its costs, and will likely be better off with humanist instead.
Edit: Forgot better relations over time.