Timing is key of course.
There are times when it's in there for a long time, just due to the nature of the new units becoming available.
Also, once you start combining units, the maint costs drop somewhat, making the reducing card less valuable. (less units)
At that point, the upgrade card is more valuable.
Early on, when gold is low, cutting maint costs is nice, sometimes critical.
Then again, faster build is also important (so ya don't get kilt)
So, it all depends. What government, did you have any of the extra slots? If you can get all 12 slots, then at that point, it doesn't really matter.
with 4 wildcard slots, I'll use the GM/GS cards and that's it.
I've got enough Hansas that I don't need the GE card, don't care about the culture ones anyway, GG/GA I'll get some, but meh.
GS for eurekas and fast science, GM I want the extra policy slot and some of the amenities. GE I really want the ones for pushing wonders.
Military, I'll keep the fast build for land units, faster encampments (I always seem to have some under construction), others I'll swap about.
Political ones, faster envoys, then whatever, I'll mix that one up a fair bit.
ymmv of course.
