The 840 Pro is the better one, although the performance difference is small compared to the 830. The E: drive would be my choice to put the SSD to work.
Have you considered keeping the 830 for previews only, adding another volume to your system and raiding D: with one of the conventional disks you are replacing?
I'm actually working from a laptop, so 4 drives (with 1 being the mSATA slot) is the max I can do. I have considered setting up a RAID / keeping the 830 though, but I think I'm just content with just replacing a single drive and not setting up a RAID just yet.
I have never seen a laptop with three eSATA ports to accomodate all these external disks. Care to give some more details? Like connections used, any partitions (heaven forbid) in use, anything other relevant?
Sure. I'm using a Dell Precision M6600. It has 2 HDD slots and I removed the DVD drive and replaced it with an HDD caddy for the third slot (and of course it has an mSATA slot for the mSATA SSD). Everything is internal and no partitions of course.
I could technically have more disks I guess with the (1) eSATA or (2) USB 3.0 ports, but I didn't want to have to attach externals.