What's more important in a weapon, damroll or average damage? When two weapons are fairly close together in damage, say, separated by 5, but the one with the lesser damage has better damroll, which would I be better off choosing if those are the only factors in my decision?

Well, there are other factors here. First, there is the level of the weapon compared to your level. If you are level 20 and have a choice between a level 52 weapon that has avg 28, or a level 25 weapon that is avg 21, I would use that level 25 weapon as you wouldn't get that pro-rated damaged reduction.

Secondly, you need to consider whether or not you have enhanced damage. If you have enhanced damage, then the damage bonus fires off of the average of the weapon and will really outpace any +dam bonuses in most cases. If you do not get enhanced damage, I probably wouldn't bother with either +dam or weapon averages and try to concentrate more on getting a weapon that fires off special progs or that boosts your HP, saves, or other critical stats since you will get a lot more mileage from it.

Thirdly, some weapon skills seem to do damage based upon the average of the weapon, so really high-average weapons do a significantly large amount of damage compared to just having a high dam roll.

And finally, you need to take vulns and resistances into consideration, especially stacking vulns (like a silver wrathing weapon versus undead), since adding 33% or more for each blow can literally light your target on fire if they are not able to defend against it.