GJ...I'm not from Texas...close by in NM...and hunt Rios there pretty much every spring...as well as Merriams here....so I will give you an answer to your question about the difference between Merriams and Rios from my perspective.
Overall size-wise, I think they are pretty comparable. My experience is that mature gobblers of either subspecies average about 18 pounds from the areas I have hunted. I have seen both in the low 20's on the upper end and some in the 16-17 pound range, as well, on the lower.
In terms of beards and spurs, Rios generally have Merriams beat in both categories. My opinion is that, when comparing birds of the same age, Rios will generally have heavier beards that average 1/2" to 3/4" longer than Merriams, and the spurs will be better developed, sharper, and average 1/8" to 1/4" longer.
Of course, these variations are based on my experience with birds from relatively small regions. I am sure all of these factors can vary considerably based on genetic and nutritional differences in different parts of the country. However, I do feel that Merriams, in general, are genetically predisposed to having less developed beards and spurs than are Rio Grandes.