[Luinadar]

It probably has less to do with power - how do you measure that kind of mystic strength anyway? - and more to do with motivation, will and ethics. Sure Gandalf could go as apocalyptic as Sauron, but he never would as it wasn't his nature. To emulate Sauron in a flagrant display of power would be evil, period. That is why The Dark Lord is so feared and revered, he has no morality to keep him in check and therefore no real limit to the horrors he is capable of. Saruman however could have been a realistic corporeal threat to Sauron. Sauron himself most likely knew this too, and therefore enslaved the will of Saruman so as to remove his only possible peer in power and darkness.