The above is right, USB itself (if it is 2.0 or above) has no noticeably worse latency for most applications than PCI.
RME (big wigs of digital audio hardware) make a 'fireface UC' model which claims revolutionary low latencies on USB 2.0
Latency issues are generally more due to issues with the compatability of the model itself and your USB port, drivers and how well your computer is setup for audio, which will heavily affect what buffer size you need to set.
I've spent a long time with my (heavily) outdated computer setting it up for optimum performance and while recording, I generally run at 64 samples, which gives me about 2ms latency which is more than unnoticeable. This is with a simple dual-core setup, 4GB Ram and an 8 year old (at least) motherboard...