Both are very similar with min. yield stress of 50ksi (50T has higher range though 50-80ksi compared to 50-75ksi of 50). From structural point, only 50ksi is allowed to be used in the design so basically same. However, 50T has higher tensile strength of 70ksi than 65ksi of 50. Both are primary steel used in critical places (like jacket major joint can etc.) with all kinds of supplementary requirements.
Hope this helps.