Effect of using SATA 2.x and 3.x in zfs mirrored config

Hi all, I installed FreeBSD "root-on-zfs" using the installer on two new WD Blue 1TB SSDs (mirrored).

Looking in dmesg, I see one of the drives is using SATA 3 (600MB/s transfer), and the other is using SATA 2 (300MB/s transfer). Will mixing SATA 2 and SATA 3 drives cause any technical issues? Is there any reason why I should use SATA 2 for both drives?  

The system was an enterprise castoff circa 2011 HP 6200 Pro Microtower (i7-2600 CPU). I naievely thought there were four identical SATA ports and plugged in the two SSDs, a rotating drive, and a DVD.  However, looking now in the illustrated parts guide I see only one port is SATA 3.

SATA0 1st hard drive (SATA 3.0)
SATA1 2nd hard drive (SATA 2.0)
SATA2 1st optical drive (SATA 2.0)
ESATA eSATA connector

From dmesg:

ada0: <WDC WDS100T2B0A-00SM50 415020WD> ACS-4 ATA SATA 3.x device
ada0: Serial Number 21401J801472
ada0: 600.000MB/s transfers (SATA 3.x, UDMA5, PIO 512bytes)
ada0: 953869MB (1953525168 512 byte sectors)

ada1: <WDC WDS100T2B0A-00SM50 415020WD> ACS-4 ATA SATA 3.x device
ada1: Serial Number 214036801110
ada1: 300.000MB/s transfers (SATA 2.x, UDMA5, PIO 512bytes)
ada1: 953869MB (1953525168 512 byte sectors)

ada2 at ata2 bus 0 scbus2 target 0 lun 0
ada2: <ST31000322CS SC13> ATA8-ACS SATA 2.x device
ada2: Serial Number 5VX2H84K
ada2: 300.000MB/s transfers (SATA 2.x, UDMA5, PIO 8192bytes)
ada2: 953869MB (1953525168 512 byte sectors)

cd0 at ata0 bus 0 scbus0 target 1 lun 0
cd0: <hp DVD-RAM GH80N RF03> Removable CD-ROM SCSI device
cd0: Serial Number 217CD060574
cd0: 150.000MB/s transfers (SATA 1.x, UDMA5, ATAPI 12bytes, PIO 8192bytes)
cd0: Attempt to query device size failed: NOT READY, Medium not present - tray c


Thanks,
Dale