.
More reasons to build Mech instead of Mot and Armor:
1) Mechs have a move speed of 10 (Mech-E have 12!) while Mot have only 8.
2) Tanks are incredibly expensive. Typical quality tank (basic medium 70mm+ or better) costs 18 IC while a mech costs 9 or 10 IC (these are base costs without brigades). As a side note, brigades seems to add a fixed amount to the cost so brigading your armor makes complete sense. For example, normal INF costs 6IC, INF-A costs 11IC, normal armor costs 18IC, ARM-A costs 23IC, much higher brigade cost for the inf on a percentage basis.
3) With a solid committment to artillery and armor research, the stats of your mech divs will approach and oftentimes exceed those of your tanks later in the game. Tanks must be upgraded to benefit from research advances; mechs do not.
Now I will treat you to a harangue about my personal army construction doctrine

First a comparison of units (side note - when I say 'rules the battlefield' below I mean discounting air for the moment. As of the latest patch 1.06 if you are being dominated in the air you are in deep trouble no matter what land divs you decide to build):
MECH - clearly the ruler of the battlefield in the mid-late war years. However, if you haven't researched strongly into AT guns, tank destroyers, and assault guns (in the armor and artillery trees), tanks are superior.
ARMOR - Great at all times once you get at least basic medium tank 70mm. Tanks rule the battlefield in the early war years. But they are very expensive IC wise compared to all other units.
MOT - the only advantage over INF is the 8 move speed. MOT are soft targets, which means they will take more damage in a battle and will cost you extra supplies to reinforce them over INF because they cost more to build. However, MOT are useful for moving into conquered territories quickly for defense and other defensive operations such as sitting behind invasion beaches to move in fast to bolster beach defenses. Once you have 'mechanized HQs' technology and can build MECH you should never build another MOT div.
INF - Inexpensive to build and needed for difficult terrain attacks that the fuel-guzzlers can't handle.
COMMANDO - Marines, Mountain troops, Paras, etc. Obviously these can be treated as INF above except that Marines can actually effectively attack urban terrain so if you are Germany and need to capture urban centers, building marines is a great idea.
So what do you build?
MOT - These are just not worth the cost unless you are a nation with lots of MP to burn such as USA or USSR. So you should not be building MOT at all, except a few MOT-E divs to give any existing MOTs you started the game with some river-crossing ability. If you are USA or USSR you can build MOTs as long as you do not have the tech to build MECH yet.
ARMOR - Do you have basic medium tank 70mm or better medium tanks researched? If so, first upgrade your old tanks, and then at those times when you have a large surplus of ICs build tanks. If not, you can build ARM-E tanks that you leave in your queue until you can upgrade them and then immediately upgrade them. If you have the tech to build MECH you should sharply curtail your ARM builds in favor of MECH.
MECH - Obviously once you get 'mechanized HQs' your first thought when building new land divs should be MECH. I prefer to build 11 divs of mech and then add in a ARM-E to make a 12 stack with river-crossing capability. Or, make a 9-12 stack of all MECH-E for a fast army corps with unmatched speed.
INF - Early game and dont have your basic med tank or mech HQs yet? No brainer. Primarily INF builds.
In closing I want to thank all the talented opponents at HoI PA (Players Association) for teaching me to always think hard about how and what you build, and then rethink it again. I am sure many of those players, who btw are better then me and kick my a** regularly, would disagree with some of what I say above but I think fundamentally it is sound.