If portability or compactness are important issues, it's good to realized that the are a variety of very competent compact devices out there nowadays.

But that tiny, USB-bus powered devices must have certain limitations. For one thing, the "full sized", self-powered DAC have the potential (at least) to have superior analog sections -- and no DAC is better than its analog section. Tiny, bus-powered devices must rely on op amps for their analog sections. Also, with tiny devices there is less scope for including higher-end USB asynchronous receivers or multiple DAC chips or such amenities as multiple DAC chips. And obviously the provision for multiple digital inputs isn't likely.