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Went into full anal-retentive mode for the benefit of those undecided on which Rex to buy.
I put together this scaled comparison of some of the available Rex figures to see if there are indeed any differences in proportion as some seem to believe (left side of pic). No surprise here, they appear to be the same. It's only cost-effective for Mattel to source a single sculpt, likely a digital model, then size it up or down as needed.
They do, however, have different poses which affect how lifelike they appear. The TnT naturally has the torso and head drooping down, giving it the impression of heavy weight. You can almost picture it lumbering around, bobbing as it goes. This is likely due to the big, loose joints employed in the thrashing action. The EC will not have the torso joint, but it does have ample neck articulation so that you can simulate a droop at least in the head. The SC doesn't have the droop, so it feels more stiff.
Another note: the paint apps seem to differ between the TnT and EC (right side of pic). TnT has darker browns on the nose and around the eyes. IMO, this contrast makes for the superior paintjob of the two, but if you're planning to repaint your Rex, this doesn't really matter.