I always prefer to consider facts rather than rely completely on theory. Some years back I conducted a test of SAN RAID 5 disk arrays vs. RAID 10 arrays, for various sizes and combinations or read and write activity. I found that as disk size increased, the performance of the RAID 5 arrays caught up to the RAID 10 arrays, and by the team I reached 512Gb sized disks, the RAID 5 array was actually faster. Unfortunately I no longer have the results, but in any case your mileage may vary depending on what hardware you are using. If you have a SAN and you have the flexibility to try both options, I would suggest that you try a direct comparison by using the SQLIO benchmark tool and also try timing some simple SQL server operations (e.g. backup).