Since this thread was not only about the Spanish ideas in a vacuum, but also in direct comparison to the Castilian (and thus also about whether it is good to switch ideas when forming Spain), let's compare the Castilian and Spanish ideas directly:
- +15% morale of armies, +1 colonist and the special ability to fabricate claims in all overseas provinces are in both idea sets.
- Castile has
-+5 marine force limit -> at best nice to have in order to be able to use sailors for manpower
- -1% army tradition decay -> weak
- +1 missionary and +2 missionary strength -> decent but almost overkill if you take religious ideas
- +10 production efficiency -> decent
- +1 diplomatic reputation -> good
- +33% heir chance -> quite worthless, even with disinheriting
- +10 % institution spread -> weak
- -0,05 monthly autonomy change -> relatively weak
- 25 global settler increase -> decent
- Spain has
- 1 artillery fire awesome
- 2 yearly papal influence very good
- +25 % naval force limit decent
- 10 % heavy ship combat ability decent
- +1 Naval leader maneuver ok
- +15 global tariffs rather underwhelming
- -33% envoy travel time quite worthless
- +1 diplomatic possible policies situational
- +1 prestige weak
- +5 discipline fantastic
The +5 discipline and +1 artillery fire give Spain a lot more of a military punch. In multiplayer, I think that alone makes the idea switch almost a no-brainer, and also in singleplayer, those two military bonuses are probably more useful than anything in Castile's set (which isn't that bad either).