Nothing in game!
Something in SE would be unfair to the ones not playing TE and would also risk to ruin this last, by making it "too serious" (as others already pointed out). Maybe a Donation (non-stackable with a bought one), but not beyond that.
Something in TE would on the other hand be unfair to the other players in the next round (actually, an handicap to the winners would be better in terms of gameplay...)
The best is something to brag about, for fun. The T-shirt, personalized flag(s), addition in forum profiles and/or in Nation pages are all good ideas.
I'd make a permanent "Hall of Fame" accessible in-game, that keeps the records of the winners of each round (maybe 1st, 2nd, 3rd - Player and Alliance). To stick with the KISS paradigma I'd make the rank depend on NS and score (for Nations and Alliances respectively).