Yes and no. I believe optical bit and sample rate is limited by the medium, USB can handle higher bit/sample rates. With optical, your PC still does most of the heavy lifting while the USB version will take most of the work load off the PC.
Not trying to be defensive about Optical (or trying to influence against USB), just giving everyone some food for thought.
Three things to consider about optical.
First, it's been a standard for give or take 30 years now. The technology is, for lack of better words, solid; Tried, tested, and true, it's been the standard in the industry. The signal is pure, free of any EMI/RFI from the other equipment, mainly your PC. USB, Async or not, has problems. Not necessarily 'jitter', but if you look at the roadmap you see how USB is connected to not only the chipset of the motherboard, but the power is also shared by the PSU/Battery with all the other components. Long storey short, it can be a real pain in the ass to get a 'clean' sound. Recently though, motherboard manufacturers have addressed the Audiophile community and now include a completely isolated and shielded gold-plated USB connection just for DACs. I personally use an isolated USB bus attached to a powered USB hub.
Second, most PCs easily handle sample-rates from 22KHz-192KHz with everything in between via the Optical out. Macs however, unfortunately top out at 96KHz (which is why I prefer USB).
Third, in terms of PC workload, my old PowerMac G4 (Digital Audio) at a whomping 733MHz could easily handle HD audio out via TosLink, taking up about 2% of the CPU. With today's CPUs workload is moot.