SSA and CPS are actually the same thing, in some fashion. I'm not 100% up to speed on the details myself, but as I understand it, CPS is the functional-language form and SSA is the imperative-language form.
If you allow first-class continuations in user programs, though, all bets are off and CPS is more expressive.
Richard Kelsey wrote the paper that talks about how to convert between CPS and SSA forms, along with the first-class continuation limitation mentioned above:
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.3.67...
[1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.34.3...