The major difference between Sylenth1 and Serum is that Sylenth1 is primarily a virtual analog subtractive synthesizer (VA synthesizer), while Serum is mainly a “wavetable synthesizer”.
However, Serum is not only limited to being a wavetable synthesizer. Because with Serum you can do pretty much everything you can do in Sylenth1.
In addition, Serum allows you to do FM synthesis (a.k.a Phase Modulation), and that’s something you can’t do in Sylenth1 since it’s only a virtual analog synthesizer.
So in terms of features Sylenth1 is pretty limited compared to Serum. But either way Sylenth1 is capable of producing astonishing sounds in the right hands, that is.