I Bring news!!
I have created a map just to check what is more powerful, +1 food/prod or a 50% discount in whatever thing.
I did it with England and Spain, as we are having the discussion with them.
The capital is in a tile sourrounded by land and with no ressources (but hills) and the second city in a flat tile AND in another continent.
None of them have ressources improved and have just 1 of population.
I'm not very good at this kind of analyses so I'm sure I'm forgeting a lot of things, variables and possibilities.
Feel free to comment your opinions and your criticisms, as will, surely, improve the data.
I'm runing other scenarios, so somethings not present here may be already tested!
We have to point some things before we conclude anything:
1) that the spanish bonus is usefull for any kind of production, not only districts. That gives it a lot of versatility.
2) the % discount is just for 1 district (not always, as england has 20% on IZ too).
3) As time passes, the Spanish bonus get less and less useful but the % discount gets better and better. So we have to assume the gap will increase as the game progresses.
4) A faster disctrict also means a faster bonus to internal trade. For example, the "4 turns "advantage that england has, will allow it to have a +1 production during these 4 turns. Closing the gap with the spanish bonus until spain make another disctrict.
(Germany would have the best of these bonuses as he can build a +1 district).
5) This is an ideal map.
In game, you will have access to the 50% as soon as you can produce the disctrict and to the +1 food/prod as soon as you build a city on another continent and you have the trade going.
I would put here a little advatage to the 50% as is difficult to the spanish to get soon to a new continent (depending on luck and map design).
6) The more the Spanish are late to a new continent, the less impact the bonus will do.
Conclusions:
I was a little surprised by the results as I though that Spain would do better, having the cities with little production.
I assumed that in conditions with no production, spanish bonus is better (50% of 0 is worst that 10% of 1), so lower the production imput a city has, better the spanish bonus and worst the 50% discount.
it seems to be like that in other scenarios I'm creating. But again, as soon as you produce more production, the bonus become less useful and the % discount become stronger.
What is true, is that the spanish bonus is better if you want to do other things apart this specefic construction. That help them getting faster to production of whatever they want in comparison with generic civs but slower than espetialized civs.
Seems to be the "core idea" behind Spain; they have their bonuses dispersed in all directions but they have a bonus that allows them to get faster to all these bonus.
In theory I find that way of playing a civ just fantastic. Really, now that I have understood better I will play with them again because this style is just too fun. But I find a big problem with it.
They will always be worst than spetialiced civs, what makes absolute sense
religious = Russia/India > Spain
Economic = England/Mali > Spain
etc
To counter that, Spain has a bonus that help it to be "versatile",so "against Russia? go for domination" "agaisnt england? go for religious" etc.
At the end, Spain will lose against these "specialized civs" in their domaines as espected, BUT also to the other "versatiles" civs that can make production costs drop (germany, england, japan, etc.)
Why?
As explained in other threads, Spain bonuses are too circumstantial. In the Scenario that I did, the situation was perfect. Spain had the second city on another continent from turn 1, but we cannot count on this to be a secure thing in other games.
In fact, having no Start Bias to continents splits make them loose a lot of turns until they reach the requirements for the bonus.
So at the end they cannot compete against versatile civs nor especialized civs, and they depend on luck to be on the top.
And even with all the luck in the world, the bonus of +1 F/P is average as shown.
So you have the perfect storm for them:
-They reach the bonus later that other civs (difficulty reaching new continents) > As time goes on, the bonus gets less powerfull (the bonus that you, already, are not geting soon).
You make a Civ, that tends to stagnate at the start of the game, to stagnate more and more as the game goes on. Making them be a "versatile" civ the other way arround:
Japan or Germany are versatile as they can reach whatever they want with lower effort but they are supposed to be worst that an spetialized civ on their spetialization.
Spain is Versatile as it cannot achieve anything in time. It's versatile, but not enough to be competitive agains anyone (I'm dramatizing a little

).
Proposal:
I think that the bonuses based on % are far better, as they are activated as soon as you have the technology and they become more important as game progresses. So i would suggest to check on the Civs that have bonuses like the spanish one (+1 to something) add see if can be improved.
I really don't know other civs now that have this kind of bonuses but I will check out of curiosity.
For Spain, the new perspective I gained for them is a good thing, as now I understand them better I really like the idea behind them, relatively unique and fun.
But I'm sticking even more to the suggestion I did in other thread to how to improve the CIV (only by tweaking a little the Treasure Fleet bonus), as I'm convinced that is what makes Spain not very competitive.
P.S: The Files are in spanish (I¡m too lazy to change the game

)
when the title says:
"
sin comercio" =
Without trade route
and
"
Con comercio" =
With trade route