Well, then the answer is simple: if you can accept "loosing" half of your total hd-capacity, use raid10 (I'd use 6 or 8 drives per raid10-span). If you want to use capacity of your drives more effectively, use raid5 (or raid6).
No matter how many disks you use, raid10 gives you better performance than raid5. Of course, you'd pay for it by "wasting" disk-space...