Okay, I looked up specifications for the Vertex 4 128GB and 256GB models. They have identical weights and MTBF values, so I was probably wrong about the two smaller drives having the same type and total number of Flash chips as the larger drive (two drives in RAID 0 still wouldn't have exactly double the chance of failure, but close enough). The 128GB drive has a slightly lower sustained sequential write in the specs, but other than that, performance should be about the same.