Recommendation for QNAP RAID level?

Hello experts,

Need some advise on which RaID level would be the best for the QNAP TS-EC1279U-RP with 16GB ram and to start 12x 4TB seagate es.3
http://www.seagate.com/gb/en/internal-hard-drives/enterprise-hard-drives/hdd/enterprise-capacity-3-5-hdd/

General usage for the QNAP is storage for a VMware ESX DR site. With one host and about 6x VMs that need to run from the QNAP via iSCSI (for now as fibre upgrade will be a option later)  when the DR is active.
2x Domain Controllers B
1x 2010 Exchange Server
1x 2008 R2 SQL Server
All Server 2008 R2 boxes

The production environment is not that busy with peak IOP of about 60 000 IOP on the SAN.

Because the QNAP will be used for DR and in a event of the site going live I would need the read/write to be excellent.. I was thinking of using RAID 10 on all x12 disk and in an event of a hard drive failure we have another 2 disk as cold spare available onsite.

The size of the RAiD volume needs to be about 20TB. I am not sure RAID 5 and 6 is a option with the heavy write penalty..

Your options and opinions would be greatly appreciated..

Thanks