T expires in 2 years, but will be waived if you have USA MS degree.

G expires in 5 years, so it should be probably valid even when you finish MS.

If your BS is in a top school in China, and the MS is in a 250th school here, I do not think it makes much difference if you apply with or without MS.

If you want to go Ph.D really, maybe you want to apply as soon as possible to save yourself some time because if you get a PHD ultimately, then the MS in the same area is useless.

But some people also want to go for MS and go find a job, or go for a MS in some other fields, just depending on personal career plan.