Both are stand alone apps or VST instruments, ProTools doesn't currently support VST as native, so you'd need to run them as midi slaves.
Reaktor is a full sound design suite, you can go really mad with it, Absynth is a more traditional soft-synth, a very powerful one, but it's not as flexible (or strange) as Reaktor.
At time of writing Absynth won't run under OSX, but Rektor will.