I worked at Sylvan for awhile (not in Houston though) and generally don't think much of their program. (It's very regimented and mostly consists of making the kids do 50 bajillion worksheets.) I think Kumon also uses a repetitive drill type method, which may or may not help depending on the individual problem and child's learning style.
That said, I am pretty sure both are franchised so it probably depends on the owner whether a specific location is good or not--and I think the location I was at was not good. I'd go to both and talk to them and see what you feel most comfortable with. There's also a place called LearningRx, you might consider as well. Good luck.