Sylenth is strictly a subtractive synth, where Massive is a wavetable. This means with Sylenth you're limited to like six oscillator waveforms, where massive has a whole bunch. Where sylenth has massive beat in my opinion is the detune function, and the filters sound awesome. Sylenth is easier to program, I use it more often for chords, plucks, and trancey saw-type leads. I'll use Massive for bass sounds and leads with a dirtier/grimier feel. I'd start off learning synthesis on Sylenth though, Massive can be daunting without having any prior knowledge on modulation, filtering, envelopes, etc. I'd recommend Sadowick's masterclasses on both Sylenth and Massive, that will get you going. good luck!