Thunderbolt or USB 3.1 connection?

[tsinsf]

I have a LaCie 2Big external RAID that contains two 7200 6 TB HDDs in a jbod configuration (I have 8 TB of photos on an external 12 TB drive, this Raid is a backup for that drive. I also have a cloud backup on Backblaze). It is a Thunderbolt 2 device. Soon I will be upgrading my computer and will need to connect this LaCie to a new 27 inch IMac. My connection options are Thunderbolt 3, which would require a TB 3 to TB2 adapter, or USB 3.1 second generation which would require a USB 3.1 to TB adapter. I would prefer to connect via USB and not use up one of my two Thunderbolt ports. Am I correct in assuming using the TB version would be no faster than the USB version?

[geneo]

Yes, you are correct. The USB 3.1 (Gen 1)  can do up to around 500 MB/s, Gen 2 about twice that. In your jbod configuration, you are limited to the maximum transfer speed of a single drive, which is about 200-300 MB/s at best.

[tsinsf]

Thank you.

[BobShaw]

Well for what it is worth, your drives may limit the speed, but Thunderbolt 2 does up to 20Gb/S and Thunderbolt 3 up to 40Gb/S.
USB 3.1 Gen goes to 5Gb/S and Gen 2 goes up to 10Gb/S.
I daisy chain a second drive (a long term back up disk normally off) off the my Thunderbolt drive.