For pure stunting I would say, wr250x cause its light. and you can get some stunt parts for it like rear pegs, handbrake, sliders etc.
but... I like my ktm 450exc, I ride it on the street with sm wheels, then put dirt wheels on for the weekends. Tried a little stunting, I can wheelie no problem, some short stoppies, got a 58t rear sprocket to try circles but I kinda gave up on that.
IMO it really depends on what type/ style of riding you will be doing with the supermoto. If you want something to do basic lot stuff on and to learn circles and what not...wr250 is wicked because its light,buy brandnew,street legal and RM makes parts for them.
I feel that a blue plated crf450 or yzf450 are your best choices for being able to buy parts, stunt, dirt, fmx, fast to escape in the city streets and you can throw them on the ground all day and pick it back up and they keep going. It is a lot of bike to learn to stunt on but just depends on your riding skill.