It really depends on the load required for these servers.
RAID 6 is a redundant set of hard disks that utilize double parity and striping. This allows for two simultaneous failures of hard disks without data loss. Disks may be replaced and will rebuild the RAID array without interruption of service. RAID 61 is a set of mirrored versions of RAID 6 drives allowing for 4 simultaneous failures. The striping allows for a great performance boost while double parity is actually a bit of a drain on the system, especially during a rebuild. The parity of the disks greatly increases read/write time due to the data being chopped up for parity redundancy.
RAID 10 has a better performance option, while using less disks for the build. It however only allows for a single disk failure at any given time without data loss. Rebuild has virtually no impact on performance. Its design contains 2 sets of mirrored drives in a RAID 1 configuration, coupled by as RAID 0 strips. It reads and writes from both RAID 1 arrays different sections of the same data chunk, doubling read/write speeds.
RAID 6 and 61 both have performance increases over a RAID 1 configuration, but RAID 0 is faster and faster for each additional stripe.
Migrating from RAID 61 to 10 could be a very tedious and time consuming matter. I'd advise to make a choice prior to data migration, as it can cause significant down time after the fact. It would be possible to run your active server as a RAID 10 array for speed, while running a RAID 6 array on your backup for reliability.
Note: The second image for RAID 6 is slightly inaccurate... generally a RAID 6 array has 4 drives, while a RAID 61 array has 8.