Well, both are among the strongest episodes of the HD era in my opinion and both have their qualities and their flaws. I think the satire is better executed in Steal This Episode, but I do like the structure of Bart the Bad Guy a bit more, which cleverly reflects the structure of a superhero movie or comic book of some sort. Steal This Episode is a bit clumsy with its use of guests stars, and Bart the Bad Guy can appear more "dated" due to his (less or more subtle) references to current superheroes movies (even if, fortunately, it's far from being the main focus), though I do believe the satire about spoilers will remain relevant for a moment.
I wonder what the results would be like if more people take part of this debate since Bart the Bad Guy was very well received when it came out. Personally, I'd say Steal This Episode too but frankly by a short margin.