I believe that in terms of raw strength, the True Power is stronger and also far more intoxicating than the One Power. Both male and female Forsaken can access it with the Dark One's permission, Rand is able to through his connection to Moridin.
Following Tarmon Gai'don the True Power and the Dark One don't disappear, they are simply sealed away as they were before. It was the True Power that led to the Dark One being released, as the Aes Sedai of the Age of Legends sensed it and bored a hole through the pattern to tap it, thus releasing the Dark One. The draw of it was that they sensed an undivided power, and seeing as how the greatest feats of channeling were achieved through men and women working together, they thought this power that was usable by both would be able to achieve even more marvels.
The cleansing was accomplished by making a conduit of pure saidar (provided by Nynaeve) and funnelling pure saidin through it. The two ends of the conduit touched saidin itself and Shardar Logoth and Rand was able to use the two different evils to basically cancel each other out. The taint was siphoned off against Shardar Logoth, which resulted in the city being destroyed.