Very nice thread, good to see why you prefer infantry.
I do however feel obliged to add to this.
There are only a few negatives I can find with using the Inft chassis. Firstly the cost for a spy attack on Inft is less than it is for Vechs and Tanks. Their is no difference in the effects of the spy attacks though, so each chassis is effected the same way to a spy attack. Also the cost of spy attacking Tanks is only 20e more which is very little considering how often you actually get an enemy to land on an op with an agent on it and they haven't put up spy protection and how little the oil costs are to convert oil in to 20e to perform the spy attack.
The cost is indeed less, but the effects are not the same. Contrary to what some believe, I can confirm that vehicles and tanks do not get spied to 1 HP, they lose half their HP. Comparing this to inf who get damaged to 1 HP, this means that this could be a reason not to choose infantry.
Another problem with Inft I know other people find is the restocking of armour when you lose some in squads. There are a lot more units in a squad of inft, so it's a lot harder to count the squads when you do lose units. This just comes down to laziness though. One way to counter this is rather than stacking squads 21 range 9 armour (which most do for Inft) is to build 3 squads range, 1 squad armour. That way when you do lose armour you can just see how few short you are of a full squad and recruit to fix it.
If you make 3 squads of range for every squad of armor your ratio is 3:1, whereas in the 21:9 and 20:10 build it is closer to 2:1. Of course it is your own choice, but in an even fight (even amount of units) the 2:1 build is cheaper both in oil cost and in metal cost.
Then the point you raise about experience.. I don't think that player had 40 squads of R5, from what I have seen he however had around 50 squads of R4.
I think you are giving experience too little credit. Infantry needs almost 30% experience more to level up.. I am a big supporter of experience, and even this E1 where I am not really playing I have managed to kill 12k units within 700 ticks, I have only had 11 range squads that have done all this killing, and I have not had to recruit armor in the last 400 ticks. The reason for this is that R4 and R5 are so powerful when used right.
Needless to say my 11 squads are all at range 5. It's just such a destructive force.. Perhaps I could have done this with infantry as well, but it would have taken longer and it would have cost me more metal and workers due to armor replacing.
In the end it's all personal preference and I do not have a clear favorite, I decide my chassis based on what the rest of the world thinks I am using. The era I did best, M1, I was infantry and managed to rack up tons of experience on them as well. And I must admit I really profited from the 2 tick build time there. However this era I have played with heavies and loved playing with those as well. So, there's no clear preference for me.
Either way, I like topics like this in which people post their reasons for choosing a chassis
I don't think there will ever be a final say on the matter, since it is mostly based on opinion.