Later IC has more inherent value due to higher tech generally being more efficient per production cost.
For an example, let's compare two nations building nothing but infantry equipment. Infantry equipment 1 has a soft attack/ic ratio of 12, infantry equipment 2 has a ratio of 15.5. Let's assume we have two nations of equal factory counts, one (henceforth referred to as 1) just doing all mils from jan 1936, and the other (henceforth referred to as 2) doing mils from july 37.
Assuming that both countries unlock Inf equipment 2 in Sep 1939 for simplicity's sake, and start the game with inf equipment 1, by the historical war start, 1 will have a total of 4,440,000 Soft attack worth of equipment, and 2 3,600,000 soft attack worth of equipment.
What does that look like by the second date, June 1941 though? Adding on the IC between the two dates, and assuming it's all inf equipment 2 (we're ignoring efficiency because we're assuming both nations swap at the same time), 1 will have 10,950,000 worth, while 2 will have 10,730,000 worth. The gap is much smaller than it appears in terms of raw, unweighted IC, 2% vs the 4% using original values. By 1943 it's 18,545,000 to 19,565,000, with the late-civ build gaining a 5.5% advantage vs the unweighted 4%.
The magnitudes will get larger with time and depending on what equipment you measure, but the overall point is that a late-civ build will have a measurable combat advantage much sooner than the basic IC values might imply, because more of their IC is in higher quality equipment.
What strategy you should pursue is ultimately circumstantial, though, with the main deciding factors being front combat width and building slots. If you're in a circumstance where you just need units to fill the line and thus quality matters less, you want more mils. If you are running out of room to build, you also obviously want more mils.
However if you have lots of worthless land to sacrifice, open building slots, and it's unlikely for your opponent to be able to knock you out, or destroy most of your military in 6-12 months, then building civs for longer will likely be more beneficial