hard drive - What's the difference between a RAID enclosure, and a JBOD enclosure connected to a RAID controller

I've currently got an Areca ARC-1222 raid controller card connected to an iStorage Pro JBOD enclosure via miniSAS, on a mac, used for video editing. I'm speccing out a PC to replace it and have become totally baffled by the RAID options.

According to the guff the iStorage Pro has an SAS expander. So am I correct in thinking that the chain looks like

pci bus->RAID controller->MiniSAS->SAS expander->SATA-HDD

What's the [dis]advantage of this compared to a RAID enclosure with the RAID controller built in, or a raid controller card connected to something like this enclosure, which is a lot cheaper, but doesn't mention anything about a SAS expander. Or for that matter an internal RAID card connected to internal disks, ahich seems to be the cheapest option of all.

A combined RAID-enclosure takes data with up to 6 Gb/s from your PC and the RAID controller has most probably direct channels to each drive so it can write the data simultaneously.

If you have a RAID controller and an enclosure and have both connected by a single cable, the controller has to send the data to write for each drive separately. This generates overhead and - e.g. for a RAID1 - decreases writing performance by 50%. Because you can't squeeze 2x 6Gb/s (=data for both drives) through a single 6G cable.

Example (RAID1):

Combined RAID-Enclosure:

                           ,---6G---> Disk0
PC ---6G---> Controller --<
                           `---6G---> Disk1

Separate Controller and Enclosure (with Expander):

                                                      ,---3G---> Disk0
PC ---6G---> Controller ---6G (2x 3G)---> Expander --<
                                                      `---3G---> Disk1

Reading performance shouldn't be different, though, as it doesn't matter from which drive it is read - both contain the same data.