The difference between a d4 kukri and a 2d6 greatsword is big at low levels, but virtually unnoticeable at high levels. But the difference between a weapon that doubles your +39 from Power Attack and high strength on rolls of 15-20 vs. a weapon that doubles your +39 only on a natural 20 is huge.

Of course, if you're a rogue, sneak attack doesn't multiply on crits, so it doesn't really matter what weapon you're using.

On average, you do the same damage over time with an x4 weapon and an 18-20 weapon, or with an x3 and a 19-20 weapon... but the weapon with the high multiplier is far more likely to be overkill, while the weapon with the large range is far more likely to do *just* enough damage to finish an encounter a round quicker than it would've otherwise taken.

Summary: base weapon damage doesn't really matter at high levels. Crit multiplier/range matter a lot for strength-based characters (with range slightly more important than multiplier due to "overkill"). Crits don't particularly matter for rogues.