My simple thoughts:
If SAS is really important to you, you may want to check for a firmware update that might allow mix & match SAS/SATA connections. I believe with some of Dell's PERC controllers (based on MegaRaid), they allow for that type of scenario. So likely you can get an update for your LSI MegaRaid controller.
However, I would go to a SATA-only setup with that controller as a price & performance upgrade. A SATA SSD array will easily out-perform a SAS array of spinning disks (HDDs). You can create two arrays of SATA drives - 1 array of SSD (I would go for a 3 x SSD +1 hot spare RAID-5 or add another SSD for a RAID-6) as your high-performance storage, and 1 array of 4TB SATA (go with the enterprise class of HDDs) for your lower-performance archival storage array.
If you're rolling your own array, you may not have to go with the higher-cost Intel S3700 drives. I would recommend looking at the Samsung 845DC PRO (great write endurance!) or the Micron 500DC as a lower-cost but still high-performing alternative to the S3700. If absolution write performance isn't as high of a priority (for example, if this is for a webserver which is most read-only), then you can go with the SSDs that target read performance, like the Intel S3500, for a much lower cost.
As a final note, I also recommend buy an extra drive of each type when you roll your own arrays. That way, if one goes out, you will have a backup right there to plug in immediately, rather than trying to source the same drive later on.