How is Straight Tomoe Mangekyō Sharingan awakened? How is it superior to normal Mangekyō Sharingan? In Naruto Shippuden, Madara tells Sasuke that he's so powerful because he could awaken a Straight Tomoe (Choku Tomoe) Mangekyō Sharingan. I wonder what is the difference between a Mangekyō Sharingan and a Straight Tomoe Mangekyō Sharingan and how is it more powerful than normal one?
Upon using Mangekyou Sharingan, the user's eyesight will gradually gets worse. The more the frequency and the more strain the user puts on the eyes, the faster the process will be. Using Mangekyou Sharingan techniques such as Amaterasu, Tsukuyomi, and Susano'o puts great strain on the eyes, much much greater than just activating it. Thus, Mangekyou Sharingan will gradually lose its eyesight until it became completely blind. The evidences are:


Itachi was almost blind during his fight against Sasuke.
Sasuke was nearly blind after the fight against Danzo.
Madara said that he was pretty much blind when he transplanted his brother's eyes.


The Straight Tomoe Mangekyou Sharingan is a Mangekyou Sharingan that won't gradually lose its eyesight like a normal Mangekyou Sharingan. Thus, it is also known as the Eternal Mangekyou Sharingan. This is the biggest difference between the two.

Another difference between the two is that an Eternal Mangekyou Sharingan (henceforth EMS) is awakened by transplanting another's Sharingan. Throughout the canon (the manga) only Uchiha Madara and Uchiha Sasuke has achieved this. Both transplanted their brother's eye and both them and their brothers has already achieved Mangekyou Sharingan (MS) when the transplantation was done. An MS is awakened by watching the death of someone dear, which in Itachi's case was the death of his best friend, Uchiha Shisui. In Sasuke's case is the death of Itachi.
If I recall correctly Madara awakened his MS after the death of one of his brothers.

An EMS is not more powerful than an MS in term of power. It also doesn't unlock more occular jutsu. Upon acquiring EMS, Sasuke's Amaterasu didn't get any stronger. EMS only restores an MS' eyesight and removing its drawback of getting blind, which is why it is called Eternal.