If you're going for a Duchy rush (..or.. estate
), then copper is a GOOD card and trashing them is slightly counterproductive. That would mean all Moneylender is, is a generic terminal silver (since the +$3 it gives you includes the value of the copper you would have gotten to play). Cutpurse meanwhile is a +$2 to you and a -$1 to the enemy. Both of these assume that they'll hit, though.. Cutpurse is better again, since it gives you its +$2 no matter what, and its copper matching has 5 chances to work instead of 4.
Also, in general, I find that Cutpurse is by far the most infuriating when I'm aiming for $5, which seems to be the case here.