For using sound card modes like PSK31 there won't be any performance difference between the Signalink-USB and more expensive interfaces. What the more expensive interfaces get you is added features such as rig control, FSK keying, direct CW keying, and WinKeyer for keyboard CW operation.
The benefit of the interfaces with an internal sound card is that it frees your computer sound card for normal Windows uses and keeps you from having to switch or swap cables between the speakers and the radio interface. You never have to worry about accidentally transmitting Windows sounds over the air. On digital modes the sound card only acts as an A/D and D/A converter so it doesn't require a very high end sound card.