Merchant Confederation (3) (6-3) - I love to get as many envoys as possible, but feel that the extra gold per turn is less impactful as being able to upgrade units for 50% gold. I think it is actually in the favor of professional army that you only need to slot it when you are actually upgrading, and the rest of the time you can run a different card. Merchant confederation has to be plugged in every turn to get the full benefit. Also, there may be a window where there aren't many other green cards available, but I like to run Machiavellianism as soon as it's available. I find once you reach that point, competition for green slots is tighter than for red (at least if you're peaceful).
Professional Army (29) (28+1) - Gold (not strategic resources) is what I find to be the limiting factor in upgrading units, and this makes a big difference. As noted above, you don't need to have it slotted much of the time to get the full benefit.