I've never encountered any treatment of attenuator topology, but if someone off the street asked me to choose, I'd pick T over Pi, because series R up front does a better job of hiding non-idealities than shunt R. Think about it: a leading shunt R demands perfection in the transmission line leading up to it, while a series R devours imperfections both prior and forward.

Not sure what you mean by topology? If you mean Tee or Pi type you may be surprised that most are neither, relying instead on a 2D slab of resistor material (which ohms out properly as if it were Tee or Pi). The big trick is broadbanding techniques especially when trying to ground the shunt. I would need to find an old paper to explain this type of attenuator. Madengr should know.