T-Mobile does not have the lollipop update for the G3. Have you not been reading all the threads from the S5 users where they have had major issues with their phones after upgrading to lollipop? And you still want that on you phone?
My brother has a G3 on AT&T and his phone has been running way better than it ever did on KitKat. It doesn't surprise me that S5 owners are having problems. Every Samsung phone I've ever had turned into a lagged out pos after updating it.