The big differences are (a) zswap is in mainline, zram is not, (b) zswap can swap pages out to disk when it's full, zram cannot.

Also, I wouldn't really characterize the difference that zswap makes as a constant, workload-independent percentage performance boost. Rather, it increases the amount of RAM you can use before the system moves from the "acceptable performance" regime (minimal swapping) to the "dismal performance" regime (lots of swapping). For things like running Rails apps on a relatively small VPS, or getting the most out of a limited-RAM machine like an Eee PC or Raspberry Pi, this is very important.