Amp vs Crit

When looking at Amp vs Crit there is only one factor which determines what is better and that is your crit rate.

The general order in how they affect your attack power is: Base attack value + (Base attack value * (%amp/100+1)+ Add attack value = End attack value. On a critical hit this is a bit different, because crit damage also comes into play, it is still easy however: Base attack value +((Base attack value * (%amp/100+1)) + (Base attack value * (%critdmg/100+1))) = End attack value. (Not 100% sure, but test I done showed me critdmg is calculated before add attack is added.)

As you can see from the way it is calculated crit damage and amplifier do exactly the same. This is where crit rate comes into play, because you don't crit all the time the critical damage isn't added on every attack. If you have to decide between, for example, A 7% amp/10% critdamage blade and a 20% crit damage blade it is depending on your crit rate which will be better.

7%amp/10%critdamage weapon.

20%critdamage weapon

Now to compare an extended weapon with 40%critdamage versus a 7%amp/28%critdamage weapon. At 50% rate, you will have an average damage increase of 20% from the 40%critdamage weapon, to compare, an 7%amp/28%critdamage weapon will give you 21% average damage increase at 50% rate. At 60% rate, you will have an average damage increase of ((60*140)+(40*100))/100-100=24% average damage increase, to compare, a 7%amp/28%critdamage weapon will give you ((60*128)+(40*100))/100-100=16,8%+7%=23.8% average damage increase.

With this information you should be able to calculate which will suit you better, but under 60% crit rate, this will most likely always be amp.