I used my first one to get the +2xp, and then the war ended soon thereafter.

As I built up for my
next war, I was wishing I had used him to build a Military Academy (+25% military production). So, lesson learned.
(Not that the 2xp was useless... far from it. My uber Berserkers really kicked butt.)
As for the 20xp usage... I'm thinking myself that the bigger benefit is the free upgrades. You get a really nice elite unit, now that guy isn't nerfed and will get better and better for the whole game. Frankly, I think this might be even better than the Military Academy.
By the way, I was curious if the +2xp version (which joins him as a super specialist) causes him to eat food, to which I found the answer is NO. Good news.
Wodan