A good primer for SSDs is to read this anandtech article.
For regular desktop use (gaming, office, productivity, what have you) SSD will kick 15krpm SAS drives around easily, since there's a lot of random reads and writes involved. You do have to be careful in choosing the correct SSD though, choosing one of the older models won't give you that much of a performance difference.
If, on the other hand, you're doing a lot of sequential read/write, like for instance editing videos and working with large data sets (moving large VM images around, etc), the SAS drives will be very fast. On the other hand, the SAS drives are just as fast at it as fast SATA 7200rpm drives.
So, it all depends on what your use case is. If you're planning for normal desktop use, SSD is the way to go. For big chunks of sequential r/w you'll be better off with a pair of spinning drives.