Both of them are really strong ideas. I always take at least one, and would sometimes take both.
Humanism is great if you can stack it with tolerance NIs (=heretics/heathens reach +3 or thereabouts and cause no problems) and/or culture acceptance bonuses. -65% or better culture acceptance threshold is insane - you end up with half a dozen cultures without even trying. It's also the best idea bar none for soothing unrest in newly-acquired provinces and will let you reduce autonomy a lot sooner than you would otherwise (or at least not have to raise autonomy when you take over). I'm a big fan of Humanist for merchant republics, England (especially if you have continental ambitions), and countries that start in area with lots of medium-sized cultures and no big culture group to call their own (e.g. Ottomans, Indians and any European who isn't Iberian, French, Germanic, Latin or East Slavic).
Religion is great if you want to go full Borg and create an ultra-stable, homogeneous empire. Like cultural acceptance, culture conversion really shines when you can stack the bonuses. For instance Muscovy with Religious ideas and the Religion+Influence policy pays -65% conversion cost, at which point it's feasible to just carpet-convert large chunks of Asia. It's a similar story with stability - if you already have a discount, it becomes very cheap, allowing you to be much more aggressive with stab hits. Religious conversion is also a must if you are Orthodox, because of all the bonuses you get from Patriarch Authority. It's a great idea group if you are a non-Sunni in Africa or Asia (so many heretics and heathens to conquer/convert, and initially no Expansion CB available), a Muslim going for 'Unify Islam', an Orthodox country, a Reformed country that plans to be aggressive in Europe, a theocracy (so much true faith tolerance!) or a Catholic country in a game where the Reformation is especially strong (more conversion needed, and the papal influence counts for more because there is less competition for the Curia).