I'd highly recommend the RAID option, especially if you can get a third 100GB drive (for one of the RAID levels with redundancy (RAID-5? I don't remember). Dynamic disks are... well, difficult to manage. everything works alright as long as all disks remain in tip top shape, but if you lose a single disk you lose all the data in all the disks (if I remember correctly). At least with RAID, you have a protection mechanism.
Also, with Dynamic disks, it's a software mechanism implemented by windows- for RAID you can often configure a hardware solution.