I prefer the "V". Spent an afternoon watching Kit Ravenshear make springs and he was amazing.
But, if my life depended on it, I'd have to go for the reliability and longevity of coil. Although, knock wood, I've only had one "V" spring break in many, many years of shooting.
Of course, ALL flintlocks should use "V" springs by order of artistic and spititual wholesomness. It would just be wrong to use a coil spring flintlock. Right up there with putting a synthetic stock on one. Ghastly.