Flame blade vs holy weapon

[occasionalplayer]

I have trouble understanding which is better under which circumstances. 
It appears that holy weapon is stronger most of the time unless the attack strength is 3 or lower. Is that correct?
Thanks for any advice.

[Seravy]

Assuming your current attack is X and your current To Hit bonus is Y,

Holy Weapon adds 0.1*X damage
Flame Blade adds 3*0.1*(3+Y)  damage.

If you want Holy Weapon>Flame Blade then
0.1*X>0.3*(3+Y)
X>9+3Y
Meaning if you have no To Hit bonus then Holy weapon is better if your attack is 10 or higher. Each To Hit bonus means you need an additional 3 attack above that for Holy Weapon to be the better one.
This is for melee, for ranged, as Flame Blade adds only 2, you need the attack to be 7+2*To Hit or higher instead.