I saw both play in their prime. Bruce had a longer and more sustained period of dominance. By Super Bowl XXV, LT was spotty in terms of an impact player. He was completely invisible in that game.
I'd even say he was less an all-around force by 88 or so, a one dimensional pass-rusher. Now early in his career, he was the most dynamic defender I'd ever seen.
In 81 or 82, he picked off a pass on the Gints goal line against Detroit and sprinted the distance faster than a halfback. Speed of sound fast for a man that size. That play still blows me away.
When Biscuit came up in 87 for those last 7 games , I actually thought he would be more dominant than LT if that was even possible.
All in all, if I were a GM, I'd take Bruce. Especially when paired with a young Ted Washington.