something u should think about, look at the audio/modulation ins and outs of subtractor, malström, thor in rack view and manual. Get it? Principally u can route endless instances of these devices to and in each other and create new combined synths capable of more than massive ever is possible. Also u have the matrix pattern sequencer, audio splitter, lot of effects which can all be routed INSIDE freely in this new created synth. But u have to get ur head around this and be creative. But this way u have a synth capable of endless filters, waveforms, modulation, fm, wavetable, graintable, subractive, phase modulatoin, multi effects......
If Reason would be loadable as VST, no question i would favour it to massive. Not that massive isnt a very nice synth, use it myself in Ableton, but dont think its more powerful than the routable Reason synths & fx. Set up in combinator u can also create key vel settings for this synth and macro knob to control it better.