Yay! A subject near and dear to my heart.
Of the two, I would highly recommend Onstar. We've seen a lot of really nice calves sired by him, plus we have a son we are using on our commercial herd. We used him heavily in the spring this year, and also bought some embryos. He passes on good lines, a little trash, depth of body, ease of fleshing. Watch for white spots though. He seems to breed to the cow on bone and muscle...doesn't really add any. Not super growthy, either.
I haven't seen a whole lot of Texas Star calves in person. The ones I've seen in catalogs I haven't been too crazy about.
I saw Stonewall in person, and will never breed anything to him. Very piece-ey, shallow flanked, not much muscle, and a really snotty attitude.
Three more bulls you might want to look at. TJM Coronado, CB Final Cut, and Suhn's Augustus. We have been using Coronado heavily since he first came out. He is our go-to heifer bull, and really changes the cow. He makes them jet black with fancy fronts, adds bone, moderates frame, gives them a fancy square hip and adds ease of fleshing. The two calves we had this week out of heifers were 57 and 58 pounds. Nothing over 75. We tried flushing a cow to him, but didn't get anything. His semen is excellent...we have had very few miss. We saw the high selling Final Cut bulls from the Kansas sale, and liked them enough to breed several of our cows to him this spring.
We recently bought some cows that had mostly Augustus calves on them. Two of them are going to be our son's first show heifers. The others are all keepers. Several of the mamas are bred back the same way, and we are pretty excited about it. He seems to breed to the cow, but makes them a little bit "better", if that makes any sense.
One more I'll mention is a bull owned by a friend of ours. GR Blanda 803U27. Super consistent and a real cow changer. Very low birthweights on his calves, as well. Has some show heifers doing extremely well here in Texas. I've seen him in person several times and he's impressive. Calves are "cookie cutter." We have several calves coming in the spring, and are planning on flushing some cows to him at some point. I have a picture somewhere if you'd like to see him.
I could talk about this stuff all day.