Here are my thoughts on why the FRA IC is the best bet.
1. It provides ‘soaking’ damage from any early game, non-dedicated SBR over a 3 IPC territory. If you aren’t maxing out all 16 production spots which you most likely won’t as you’ll be building offensive units), you can let some damage sit on your ICs as I have done in some of my games where I’ve built a FRA IC. In two games where I’ve had disastrous G1 and G2, I’ve let 7-10 damage sit on my two German ICs and I just built units wherever I could with the sub 30 (20 IPCs at one point) income that Germany was pulling in until the Russian advance stalled out and I was able to push back. Both FRA and GER were stacked so high with infantry and artillery that they were immune from attack and I was eventually able to hit back into POL and CZE while still keeping GER and FRA safe.
2. FRA is ‘safe.’ Any Axis player in their right mind will never let FRA fall to the allies. If the Allies pull an unexpected naval move or position their fleet so that GER can’t be hit (for example, moving everything to Z12), you don’t need to stack units in GER and can pump 6 units directly into FRA to defend against any possible invasion (or position yourself for a counterattack into ITA if the allies go for that)
3. The extra 3 production spots can be good to use if you are getting all 3 NOs but are not able to keep the CAU/KAR production spot in your hands for longer than a turn to build units there (SU/UK counterattacks).
4. If Germany suffers a bad first turn or two (not getting the Russian NO, not sinking the BB and losing half your airforce, getting decimated in EGY, etc), POL will be put in serious jeopardy as the Russians strengthen and push into Europe. With a naval invasion by UK forces into POL backed up by Russian land units not only do the allies avoid losing the 5 IPC NO for allied troops being in SU territory, but the UK now has a direct production spot on the mainland. Even the more likely scenario of the SU taking it and then getting reinforced by UK/US troops is a big advantage. I’d gladly trade a 5 IPC NO for a 10 IPC NO, a 6 point territory swing, and a factory on Germany’s doorstep. Russia would most likely be making 40+ IPCs a turn which would enable 10 inf to be placed in RUS/CAU to defend against the Japanese push while placing 3 offensive units in POL to finish off GER.
The only downside of the FRA IC over POL IC is that it slows down your advance into RUS slightly for one turn. However, this is mitigated to some extent by the fact that you will most likely have to send some units from GER over to FRA anyways to help defend against a turn 2 UK invasion of FRA. It must be able to hold off 2 inf, 1 art, 1 arm, 2 fgt, 1 bmb and perhaps another 2 land units, bombardment and/or aircraft depending on how GER conducted it’s first turn attacks and how successful they were. That would entail defending it with at least 8 infantry if you wanted to be reasonably certain to hold it. If you defend it with fighters, you cannot push further into the USSR if the SU does not recapture any of the BST, EPL, or CAU.