Thundurus has always been a decent rain abuser, including in Gen 5. It's not Thundurus that's worse now - its Rain itself. DAMN GAMEFREAK Y U DO DIS TO WEATHER
It's main use for Rain is to get access to STAB Thunder, which lets it hit 100% of the time in rain. Always nice. As for which one is better it's debateable. Thundurus-T has higher special attack at base 145, but lower Speed at base 101 - though that can be remedied by using Agility, and it also has access to Nasty Plot to boost it's already excellent Special Attack.
Thundurus in it's incarnate form, has better speed and attack, allowing it to function as a decent mixed attacker (Hammer Arm :P). However with it's higher speed, Nasty Plot is much more viable, and it also has Prankster which means moves like Nasty Plot and Thunder Wave will always go first. Unless against another prankster user. Or faster priority. :P