ada vs. da?

[Mikhail T.]

The four AHCI-drives in my system appear as both adaX and daX each:

    <ST3000DM001-1CH166 CC43>          at scbus2 target 0 lun 0 (ada1,pass2)
    <ST3000DM001-1CH166 CC43>          at scbus3 target 0 lun 0 (ada2,pass3)
    <ST3000DM001-1CH166 CC43>          at scbus5 target 0 lun 0 (ada3,pass4)
    <ST3000DM001-1CH166 CC43>          at scbus6 target 0 lun 0 (ada4,pass5)
    <Generic STORAGE DEVICE 0550>      at scbus7 target 0 lun 0 (da0,pass6)
    <Generic STORAGE DEVICE 0550>      at scbus7 target 0 lun 1 (da1,pass7)
    <Generic STORAGE DEVICE 0550>      at scbus7 target 0 lun 2 (da2,pass8)
    <Generic STORAGE DEVICE 0550>      at scbus7 target 0 lun 3 (da3,pass9)

Each one is listed in /var/run/dmesg.boot like this:

    ada2: <ST3000DM001-1CH166 CC43> ATA8-ACS SATA 3.x device
    ada2: Serial Number Z1F1E8NK
    ada2: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
    ada2: Command Queueing enabled
    ada2: 2861588MB (5860533168 512 byte sectors)
    ada2: quirks=0x1<4K>
    ada2: Previously was known as ad8
    da2: <Generic STORAGE DEVICE 0550> Removable Direct Access SPC-3
    SCSI device
    da2: Serial Number 000000000195
    da2: 40.000MB/s transfers
    da2: Attempt to query device size failed: NOT READY, Medium not present
    da2: quirks=0x3<NO_SYNC_CACHE,NO_6_BYTE>

What am I supposed to make of it? Can they be accessed through either 
name? What are the advantages of each? If ada is always a better choice, 
how do I make the da ones disappear -- such as from the systat's output? 
Thanks!

    -mi