Agreed, do both.
I didn't go for the Sony, but instead spent less than a third of the price on a TDK that only writes DVD+R. But I have nothing that needs DVD-R and am not likely to buy anything that needs it now.

Additionally, you can never have to much hard disk space. Somehow you will always find ways to fill it up, regardless of how big it is.

Lastly, avoid any raid setup just to get more disk space. Every array config (other than R0) will lose you at least one disk worth of space.
Raid 1 loses you 50% of your disk space
Raid 5 loses you one drives worth.
Raid 0 just spans the disks so you get full capacity, but you would be screwed if anything dies.