AM62P5-Q1: suggestions hints on ethernet PHY

Hi everybody , 

customer is needing a PHY    they are thinking on DP83TC814 .

now they need  in the system  100M  not G

they woudl like to have some hints by us  on 

- why  using RGMII  vs  RMII    if you need only 100 ? 

- 1,8V  vs 3.3 Interfacing  ?  whihc is better

thank you 

BR

Carlo