I think the 40gb option is much better.
1) You get more capacity. Even 32gb is not that much for the OS and apps. Windows-7 32 bit takes 13gb without any space trimming.
2) Raid-0 will give you better sequential benchmark performance. Unfortunately, that is not what the normal desktop user does. Small random reads and writes are the norm, and raid-0 does not help there. I changed from 2 Intel X25-M 80gb drives to a single X25-M 160gb drive. My perception was that the single drive was just as responsive, if not more so. Larger drives will have more nand chips that are accessed in parallel. Sort of an internal raid-0.
3) Some drives will depend on "trim" to keep free space available. You will lose that with raid-0. As the drive gets filled, performance will degrade. A 16gb drive will definitely get full quickly.