I like the idea of 50/50 with a couple tweaks.
First, I would make the single post random draws run for one day, do the drawing, and start another daily thread. This would give both chatty and single posters multiple chances throughout the contest period while encouraging activity throughout the contest.
Second, for the post count thread, no back to back posts would help keep it interesting.
No matter how it's done, random luck will still leave some happy and some feeling cursed. I felt cursed for most of two years, then got lucky a couple times. ECF is great for coming up with these new contests and it makes for more fun!