On your 2510, a 15.5x38 and a 13.6x38 will or at least should be the same height, depends on brand and rim width, and the wear your particular set of tires have. But going from 13.6x38 to 15.5x38 won't change a thing, and you should actually change from the 11 or 12 inch wide double bevel rims you have to 14 inch double bevel rims with the 15.5's.
As to the original post, if you like the way your 3010 sits with 18.4x38's, put new 18.4x38's on. Just because a rear tractor tire is narrower and lighter doesn't mean it will be cheaper. The volume of tires made of that size makes a BIG difference. I priced 15.5x38's for my Farmall M. They were rather reasonably priced, cheaper than the 13.6x38's like I had on the tractor, and Much cheaper than the 14.9x38's that would have fit on the rims already on the tractor. But $1000 for tires and another $750 for rims too much money for me.