First of all: SATA vs SAS drives (not discussing RPM)
SAS drives do have better error-handling than SATA, so when you have the option, take SAS over SATA, so when you have a harddrive fail the chance of killing your raid will be reduce with SAS over SATA.
Read some other questions here on SF about people loosing their raid over 1 failed sata drive.
For the RAID : you have to decide what do you actually need, reliance, nearly no downtime, fast restore , access speed, etc.
There are some nice questions here on those specific needs.
For the speed difference: 15k RPM against 7.2K RPM make a lot of difference, though this depends on the server and your data.
Basically if you anticipate a lot of IOOps and cannot afford or want SSDs go for the 15K drives.
IF you really have only a handfull sequentiell access, and no further access 7.2K drives might be enough, but you have to measure that yourself.
Depending how important every bit of data is you might want to consider the drives based on the bit error rate aswell. Keep in mind that a rebuild of a larger Raid might already trigger that bit error rate on bad drives.