The differences mainly revolve around the size of your performance. The base SSL 2 includes one high-current grade headphone output in a two-in, two-out arrangement. Jump to the SSL 2+ and you get a two-in, four-out interface with two headphone outputs for collaborative work, extra unbalanced outputs for DJ mixers and MIDI I/O. Whichever model you choose, you get a software bundle that includes Native plugins.
Both boxes are available now, with the SSL 2 selling for $230 (£150 in the UK) and its SSL 2+ counterpart going for $280 (£200). Those still aren't trivial expenses if you're just starting your career or treat music as a hobby, and they carry a premium over competing audio interfaces like Focusrite's Scarlett 2i2. They're still competitive, though, and the extra outlay may be justifiable if you revere Solid State Logic and want some of its equipment at home.