Mobile Rig - Thunderbolt or USB?

Thunderbolt (direct access to the PCI bus) isn't really necessary for audio interfaces. They mostly simply don't require more bandwidth than USB2/Firewire400 provides. Unless you're recording/monitoring a huge number if tracks at high sample rates perhaps.

Thunderbolt is useful for things like external drives. It's not necessarily that much faster than USB3 per drive in terms of data throughtput, but an SSD is better connected to Thunderbolt because Thunderbolt can pass TRIM commands to the drive, which USB3 can't. Though many external drives sold as "Thunderbolt/USB3" actually contain a USB3 drive interface with a Thunderbolt socket attached. Such drives can't handle TRIM no matter which cable is used to connect them.

What Thunderbolt provides is an easy way to add hardware that would otherwise need to be a PCI card or attached to a SATA or video controller. to laptops and Macs. It works well on Macs in my experience, allowing lots of stuff to be chained off the Thunderbolt bus. I've no experience of it in a Windows environment so can't comment on that.

For attaching just an audio interface, USB3 or even Firewire400 is usually fine. And, like USB when it was a new idea, at the moment anything with the word "Thunderbolt" in it goes for a premium price whether it's a premium product or not. :-/

USB3? You'd probably be more than OK for that setup using Firewire400.

The main advantage of Thunderbird interfaces for most people is that Thunderbird allows a lot of devices to be chained and has the bandwidth to handle the interface(s), plenty of storage media, a UAD rig and a 4K display or three. It basically does for Macs what PCIe and eSATA cards do for PCs. Often at a premium price, unfortunately.

Go with a proven (rock-solid) USB2 audio interface (RME and MOTU both offer low round-trip latency).
No current USB3 audio interface offers lower latency (current units are ironically a bit higher).
ie:  The RME Fireface UFX yields 4.3ms total round-trip latency at a 48-sample ASIO buffer size (44.1k).
 
Important note:
More bus bandwidth does not equate to better performance (lower latency or more DSP processing for EFX).
A typical audio interface with 16-24 channels of I/O is nowhere near saturating the USB2 bus... let alone USB3.
Throwing more (unused) bandwidth at the situation doesn't increase performance. 
 
Thunderbolt provides access to the PCIe bus.
Nothing more... nothing less
The one advantage to Thunderbolt (if you're running "PCIe via Thunderbolt drivers") is that you can take the ASIO buffer size down to 32 or even 16-samples.  That's sub 3ms total round-trip latency.
Thunderbolt 3 is now officially supported by Microsoft (just recently under Windows 10) 
You have to be running one of the absolute latest Z170 motherboards that offers Thunderbolt 3 via USB-C connection.
This means we finally have "PCIe via Thunderbolt" drivers for the PC.
Problem is... there are currently *no* Thunderbolt 3 audio interfaces.  
 
Apple is pushing Thunderbolt 2 for a very good reason.
None of their current generation machines have PCIe slots.
Macs do not offer Thunderbolt 3.  I don't believe OSX currently supports Thunderbolt 3.

• iMac is basically a laptop in the shell of a nice video monitor
• Mac Pro "Cylinder" (small/quiet - but no PCIe slots)
• Mac Mini

Without Thunderbolt, you can't access the PCIe bus on a Mac!
 
If you're feeling excited about Thunderbolt, go price some external Thunderbolt drives.  
1TB HD is ~$200
3-bay Thunderbolt 1 enclosure (no included drives) is $300
ie: I've got 8 drives (6 SSDs and 2 conventional HDs) in my main workstation.
That would cost $2000 to $3000 to duplicate via Thunderbolt.
 
Bottom line:
While greater bandwidth always sounds better in advertising verbiage, it doesn't tell the whole story.  
Choose an audio interface that's known to be rock-solid... and you'll never give it a second thought.

Here is 16 channels (using two of these Tascam interfaces) XLR.
Since each interface has 16 channels, 8 XLR, 8TRS, feed the XLR 8 channels of one feed into the TRS.  That is why this guy has the two 1/4" wires going into the front of one unit shown in the picture:
http://tascam.com/product/us-16x08/getting_started/
 
So $500 for 16 channels.  reasonable for sure.  Will it work?  I don't know, but it looks like it does for the guy above...