Because dimension is almost last on the list of what makes a decent intercooler core. Having just a measurement tells you nothing about the number of internal channels, if the internal channels have externals on top and bottom or the plate instead, if the internal channels have turbulators, etc. The flow rating provided by each company is unlikely to be a number they got from testing. In addition, just having a flow rating doesn't tell you at what pressure drop or efficiency, and different manufacturers rate their cores with very low efficiencies, in order to appear to have massive flow, ala. 1000 HP Cores on Ebay which aren't even close to that capable.
If you want a good idea as to the core flow, from a very conservative perspective, check the Bell Intercooler site. (Warning, it will resize your window.) They have the 24x3x10 core listed as having actual flow of 406 CFM, and the 28x3.5x10.5 as 452. But see, Bell has their core flow rates from a tested 1 psi loss at 10 PSI flow. So, double flow PSI and you essentially double flow, but at increased drop across the core. Who knows what flow rate these companies you have listed are using to measure, and in essence, it doesn't matter.
Now, onto your specific dillemma, the more important number out of the three is actually, the height(10,10.5 in this case) and here is why. Lets say that the 24" long core, cools at 75% efficiency. Adding another 4"(up to 28) is not going to give you an equal improvement. In other words, if 24" = 75% adding 4" (1/6 of the original length) will not increase efficiency by 1/6. It will be exponentially smaller and smaller until there will be no amount more cooling which can occur. So the 24 is probably long enough, and the 28 isn't going to help to any degree worth squeezing it into the bumper.
The thickness, (3, 3.5) sort of uses this same principle, but at high speed becomes something that does matter. For a car you plan on running very consistently at high speeds, by all means, get a thick core. On a street car, 3" is about all the thickness the air goes through, and by the time it reaches the back, it's already hot. So 3 or 3.5, really a wash here. The only difference is flow and only by a small amount.
The height, if measured in identical core manufactures, dictates total frontal area and therefore, total cooling. It also is a measure of total internal channels, which as I said at the beginning, is very important. However, since we have no idea who makes the cores for these companies, how many channels they have per say 5", or anything else about them, this measurement is now meaningless. For all I know, the ETS has one internal.
So, Since these are both reputable companies, I would base my decision not on the technical aspects, since you can't know them, because they are never provided in reality, because there is no standard, but rather on my pocketbook. This is as most people do because almost no one actually know jack about intercoolers.
/brox
