I was fortunate, I guess. I've scrapped Studio One 2.x (v3 is NOT to my liking), but still have the upgraded Melodyne Editor. It was only $149 for 30 days after I registered the Essential version that shipped with S1 v2. That's one thing that made SPlat so appealing to me...its ARA implementation! And then we got Drum Replacer! Good choice on my part.
One thing that's a bit confusing...
Even the Editor version comes in a file with SINGLETRACK in the file name. That is to differentiate it from the STUDIO bundle, which basically adds a DAW I clearly didn't need...or want. It does NOT refer to MONO vs. POLY.
The big jump in functionality is adding polyphonic. You will be able to use it in FAR more situations than either Essential or Assistant.