Massive is a great synth, i've used 5 instances of it for various bits on my last choon & with the whole track playing in full in barely touched 5% on my CPU monitor. (is a core 2 duo hackintosh running logic )
the routing is actually very east to get to grips with once you have been shown how to route as it not obvious. the slynth 1 is a nice synth , but the new free synth from CM music magazine sounds just as good if not better for alot of things,
of course all synths have their own character so if you can get both then why not get both.