* Ultegra is longer lasting and lighter then the 105, the materials used are more highgrade
* Dura-Ace is significantly lighter then Ultegra but sorta equal in durabilty

And be aware that a carbon frame does not have to weigh less then an aluminium frame. There are some very light alu frames out there. I think if you look further you should be able to find a good (ultegra) bike starting at 2500 aud.