RexT makes a good point about the weight to power ratio on the 3130. Kubota uses basically the same frame size from the 3130 to the 4630, a 15 HP range (you only gain 200 lbs from the 3130 to the 4630, most of it engine). NH and CIH use two frame sizes in the same horspower range, what they call Class II and Class III. The size of these two "classes" are very different, with the class III being 400-700 lbs heavier and a 9" longer wheelbase than the class II. Therefore, you could make the claim that the 3130, with the same frame size for "class II and III", is underpowered for it's frame size. If the Kubota dealer is steering you away from the 3130, I would be concerned, but who knows maybe it has plenty of power to get the job done.
Also, Deere is doing the same thing with their new 3020 and 4020 series as Kubota has done with their L30s. That is, they use the same frame size for a large horsepower range. For instance, a 3120 (29.5 hp) rides on the exact same frame as the 3720 (44 hp). It makes sense, utilizing economies of scale to cut costs. On the other hand, this may affect performance on the lower HP models. Keep in mind, this is all theoretical.... does anyone have any thoughts?