Weight and that's it. OCLV 110/120 refers to the amount of weight per sq meter (110 grams vs 120 grams). So basically you same 10 grams per sq meter. Since the only difference between the 52 & 55 is components. The 52, 55 & 59 all have the same frame dimensions, angles, etc. So it's just about weight.
If you were to paint the 52/55 and 59 the same and build them the same then I put money down that you wouldn't be able to tell the difference. When you test ride a 52, 55 or 59 what your feeling the difference is due mostly to the components and their weight/stiffness differences.
Now if you really want to discuss differences in ride quality then look at the Madone. That uses 110 OCLV and since the design is different you'll notice more in ride quality there.
Also remember that carbon isn't the same with all manufacturers, for example Giants ride a lot different than Trek. This is due in part to the different carbon used and also different design, angles, tub lengths, etc.
Plus one great thing about Treks compared to other manufacturers is that you can repair a Trek frame and for a resonable price. All the others are made in one mold so if you crash or it cracks you're out one frame.
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