Performanced wise RAID 10 is best do to the number of read and writes, plus it provides redundancy. But you may see added performance gains by having the additional drive to placxe you TL on (again 10 is preferred becaues of the stripping). But consider cost first and for most. If you will need extra space and are strapped for cash most times then plan to use RAID 5 (or even multiple RAID 5s) instead to get the performance of stripping with redundancy this will give you long term size. If you can get the hardware you need when you need it then aim if possible for multiple RAID 10s, just make sure you can justify the cost.