treditioanl VS SSD

hi,
in what way will a SSD will be faster than a seagate 15K.4 SAS or SCSI traditional drives??
the storage will be for OS (Vista) running app's, and reading media files for Avid and Adobe editing softwares.
will also run CS3 in great deal of the time.

this storage will not use for a server usage!

now i read that the SSD will run OS much faster than a traditional HD, but where will i feel it's down sides??
as far as a general OS preformance, running app's like Avid and CS3 photoshop.

i don't really know how the OS behaves in general in order to know if overall the downsides of the SSD will effect the general speed.

if not SSD, i will raid two seagate SAS drives (15K.4 SAS) which i know is fast.

thank you.