Is it better to use a SAS Expander backplane for 12 drives via one SAS connection or is it better to use a SAS backplane with 3 SAS connections? This is in terms of performance, rather than expansion. This array will be setup using ZFS on a OpenSolaris via a LSI SAS controller as an iSCSI target.
Usually one of the Mini-SAS ports on an expander is for Failover/JBOD. Your best performance is to split the disks across a couple controllers (even though they say they support 128+ devices).
You could also set up MPIO if you use two of the same controller (that support MPIO).