The slowing down is a myth. If you don't want to throw away the 150GB drive and if you think its not slow enough to put an OS in it, then go with your second method, this way you can use both drives.
When the OS has a dedicated hard drive, it's only tasks are to seek, read and write as the OS demands. Can you please explain how performance is not at all effected when the same read heads and platters are also tasked with the additional seeks, reads and writes of data storage and access?
Also, I've yet to find anybody who is not satisfied that their OS drive is "not slow enough."