RAID can improve the transfer rate of the drives, but not the access time. It's the access time you pay big bucks for when you buy an SSD, so I'd pay more attention to any difference in access time between these drive, not how fast they can transfer large files.
Your insight prompted me to look it up.
X-25M 80G = 85 microseconds read / 115 write
X-25V 40G = 65/110
Random IO per second:
X-25M = 35000 4k reads per min / 3500 4k writes per min
X-25V = 25000 reads / 2500 writes
The cheaper drive is faster in terms of latency but slower in terms of random IOs / sec...