I'm probably gong to be busy/sleeping for most of the discussion period, but I'm going to list why I think he should go to the Jades while I have the chance.
1. The arguments stated in the update - Reiklands shunt, magical infrastructure and prospective breadbasket status offer extremely unique opportunities as a ruler he frankly wouldn't have anywhere else. Uniquely and straightforwardly virtuous, as well.
2. He would swing the Jades away from the hardcore druidic traditionalists - hard to argue men shouldn't be doing green magic with the face of wizardkind in your lecture halls, and every wizard in the Empire looking
real hard at your lecture halls.
3. Popularity with the common people - the Jades, being the wizards who do food and medical miracles, are some of the most widely accepted wizards in existence, especially in the northern provinces where they work extremely closely with the cults of Taal and Rhya. A good magical and diplomatic reputation is essential for someone who will be the face of, essentially, a human rights movement.
4. This is debatable, but personally - in terms of mindset we're encouraging - I would much prefer compassion and growth to be foremost in a rulers mind, rather than intense passion or overwhelming logic.
5. Popularity with the troops - being known as the leader who directly saves his men's lives, on their deathbeds and off, would be invaluable.
6. As
@Critian Caceorte points out, and I can't believe I missed this - the Jade wind would also improve his chances of, well, living. Spending time on the battlefield or otherwise, anyone would be much, much safer when they can regenerate from otherwise fatal blows.
The Brights, who have a reputation as magical artillery and only one or two spells involving leadership, would be an autopick if they were more Indic, but they're just not. They're my second choice, but only because it offers few concrete disadvantages. He would be the perfectly spherical in a vacuum Wizard Count Fighty McFightman.
The Golds are a no go for one reason imo- Feldmenn being a politician is seen as both notable and indicative of not going far into his wind. Mandred
has to be a politician. If we sent Mandred to the Golds, we would be accepting both him not being taught the normal way, and him never going very far with his wind.