I get rather different numbers in-game.
It's now January 1939. With 1938 techs in Infantry and Armor (except for 1936 techs in Medium Tank Gun and Armor), I get:
Mech (MECx2/TDx2):
Soft Attack 15
Hard Attack 16 (total 31)
Defensiveness 21
Toughness 23 (total 44)
Softness 51 (but this will drop below 50 with the 1940 Light Tank Armor tech)
Speed 7
Air Defense 4
Supply Consumption 4.66
Fuel Consumption 6.90 (total logistical load 11.56)
Men 8,000
ORG 40 (which will improve once I've finished the 1939 MEC ORG tech)
ICs : 17.8
Days : 202 (but both cost and time will be sharply reduced once I've built a dozen or more of them)
Armor (ARMx2/SPAx2) :
Soft Attack 18
Hard Attack 10 (total 28)
Defensiveness 17
Toughness 25 (total 42)
Softness 45
Speed 7
Air Defense 3
Supply Consumption 5.32
Fuel Consumption 9.10 (total logistical load 14.42)
Men 8,000
ORG 42
ICs 17.44
Days 165 (but I've already built twelve ARM and six SPArt, unlike my zero-gearing MECH)
... so it looks like the Mech have roughly equal total firepower but more equally balanced (so my opponent's Softness/Hardness plays no role in combat), roughly equal total defense (but again, well balanced between Defensiveness and Toughness, so the unit can play either role in combat with equal success), 33% better Air Defense, equal speed, and 20% lower logistical load... I had actually expected it to be even lower, but it's still a saving over Armor.
Note that once I gear up my MECH production, they should be no more expensive... and perhaps cheaper... than ARM/SPA.
EDIT: Another point to bear in mind...
Since the main limit on the scope of Operation Barbarossa is not how many forces you can build, but how many you can supply deep in Russia, the key question is not whether a MECx2/TDx2 is better or worse than an ARMx2/SPArtx2... it's whether FIVE MECx2/TDx2 are better or worse than FOUR ARMx2/SPArtx2.
And since BWA (Battle-Winning Ability) is proportional to the SQUARES of the number of forces available (all other things being equal), the ratio to bear in mind is not 5-to-4, it's 25-to-16.