I'm of the opinion (and the math here supports it, IMO) that larger crit ranges are generally more worthwhile than larger damage multipliers... They come out even on an average damage basis, sure, but they each have their negative side-effect: Large threat range, won't always hit on a threat (dropping the average damage gained by the crit); large multiplier, and your damage potential often gets wasted on overkill. In my experience the overkill is worse than the lost threats, esp if you're assuming a fighter-type (where a 17 will almost always hit whatever you're fighting)... For non-fighters, you may need to roll high to hit, and probably won't have as high a damage bonus (meaning less chance of overkill), so the higher multipliers might actually give you more mileage.
That said, I still go for higher multipliers in practice, because they're just more fun when they happen.
(BTW, if you're curious about the effects of imp crit and keen:
Falchion v Scythe (base average damage is identical, n):
Falchion 12-20/x2: 11n + 18n = 29n
Scythe 18-20/x2: 17n + 12n = 29n
They don't change the net damage potential of a given weapon.)