My research has brought me to a site and engineers, especially this guy named Vin Curigliano, who really know their stuff regarding measuring Round Trip Latency (RTL) and having developed a true measurement tool so that musicians can have real world, practical numbers to evaluate when choosing sound cards for recording. This project (link below) is slightly over 3 years going and has just been updated with the latest results.
It's unfortunate that so many sound card manufacturers do not truly understand low latency and the real musicians' needs. What they advertise and market as "low latency" has been proven to be false or massaged numbers (see report below). IF we continue to share this knowledge and benchmark, it can only do good for musicians everywhere.
For what it's worth, USB 2.0 is generally the worst medium for professional recording. It's the slowest and can have electrical current issues that affect the sound. Occasionally, you'll find one that is okay if you are only recording a couple tracks, but if you do many live inputs at the same time, you need a superior interface. To my knowledge, no sound card manufacturer has created an external device utilizing USB 3.0. 'Have no idea why not. PCI is superior, but is now considered legacy technology, being replaced by PCI Expresss (PCIe). The companies whose cards are the best use PCIe (RME, Lynx uses Thunderbolt) and these cards are at the $1000+ mark. I just discovered, from the list below, that there is a very good, budget PCIe sound card for recording musicians by the company ESI called the Julia XTe. Can be had for $200 online.
When looking at the chart in the link (below), realize that the best card is at the top, and sets the curve for all the others, i.e. RME's HDSPe PCIe card is illustrated at 10, or 100%. The following link is part 3 of 3 of the project, so you'll want to go back to part 1 (link at the bottom of that page) and start from the beginning. Read and enjoy:
DAW Bench : DAW Performance Benchmarking
And here is the definition of RTL and the refined tool to measure it:
http://www.oblique-audio.com/free/rtlutility