Wow. I can't believe I goofed on how many years 72 months is *facepalm*. $20K / 72 months is $277 but the drawdown period is six years, not five, leaving $5,345 behind.
OK, so, recalculating, and assuming for the sake of convenience that each car is worth 50% after SIX years:
Used 10K car scenario: $5,000 + $10,000x1.07^6 = $20,007
New 20K car scenario: $10,000 + $5,345 = $15,345
Or, envisioning this the way I stated in my addendum, i.e. whether you use an income stream to pay the loan or add to an in investment...
Used 10K car scenario: $10,000x1.07^6 (initial money in the market) + $5,000 (car value) + $2,467 ($277 p/m for six years, compounded at 7%) = $44,667
New 20K car scenario: $20,000x1.07^6 (initial money in the market) + $10,000 (car value) = $40,014
Either way, roughly $4,600 difference in wealth between the two scenarios if all my assumptions were true.