The biggest difference you're likely unaware of is having to worry about having the right driver installed for the HBAs with SAS.
You likely already know that the SAS system can't support as many hosts as the iSCSI can.
How much will the iSCSI be slower with a 10G switch
The difference would be inconsequential and minimal, if you use a 10G switch that's appropriate for iSCSI in the first place (has a non-blocking backplane, supports flow control, has 9MB packet buffer w/ dynamic buffer management - the Dell Storage Compatibility Matrix is a nice guide on choosing an appropriate switch).