... will a USB interface give me good enough quality?
This is not a matter of sound quality. USB and Firewire both transport digital data and at this stage it would be too late to deal with the sound itself. The quality comes from the performers through instruments, microphones, preamps, probably additional hardware, and finally to a certain degree from the A/D conversion. Afterwards, talking about USB and Firewire, we talk about transporting data using network technology.
Manufacturers of top gear tend to use Firewire to transmit their converted data because they think that people who buy such an interface may have a lot of data, where Firewire is an advantage. Another point may be that we have many USB devices today, people are forced to use USB hubs and those boxes are not always reliable as we know.
USB is a rather "loose" connection technique, actually the most flexible networking technique we ever had for our computers. As a downside, we can easily construct little networks which work only accidently. Most of the time? Sometimes? Only with "this" plug in "this" hub? We all have our experiences.
If you think that you have your USB network under control and don't want to transport many channels of audio simultaneously, maybe in both directions, there is nothing to say against USB. And some manufacturers known for the good quality of their devices already offer USB audio interfaces.
In other words: A crappy USB interface would not sound better with Firewire and a state-of-the-art Firewire interface would not sound different over USB. It is a matter of data traffic and integration into your setup.
