When comparing Lollypop vs Sayonara, the Slant community recommends Sayonara for most people. In the question“What are the best audio players for UNIX-like systems?” Sayonara is ranked 7th while Lollypop is ranked 12th. The most important reason people chose Sayonara is:
It is so flexible it respects your local theme, or can be set to a theme, very slick, independent dark mode. And yes, it does show your cover art!
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Super intuitive way of organizing and browsing albums and artists
You can view all the albums you have in one long list. Clicking on one of them will bring a popup at the bottom of the screen where you will see a list of all the songs on that album.
Pro Works fast and reliably
Pro Super flexible layout
The latest version has a good full-screen layout and is very responsive. Goes very smoothly from full screen to minimal player (also being a GTK+ app).
Pro Integrates with the MPRIS sound menu
This applies for most players in the "Linux world", but not for all. Therefore I consider it worth mentioning.
Pro Huge development
There is constant improvements, librem and gnome3 is possibly the future.

Pro Cloud music
Lollypop allow you to play music from the web (iTunes charts and search from Spotify).
Pro Integrates well with many desktop environments
One example being the conditional use of client side decorations (a.k.a. header bars) depending on the currently active desktop environment.
Pro Good party mode
Pro Clean, light and works very well.
The only one that can play a lot of files without to stop.
Pro Queue option lets you change what's playing on the fly
You can add songs to a queue, and then re-order or remove songs as you please. It works similarly to a temporary playlist.
Pro Online radio integration
The newest version features a nice interface for adding, browsing, and playing online radio stations. This gives you access to more music than you would normally have, which can help expand your music library for free.
Pro Looks really cool, and does have cover art
It is so flexible it respects your local theme, or can be set to a theme, very slick, independent dark mode. And yes, it does show your cover art!
Pro Closes to panel while playing...
It closes to the icon (tray) on your panel, while it keeps playing. Clicking the icon you can open it again, or pause/ stop/ play it there. Great to get it out of your way when you're working with cool sounds in the background.
Pro Does conky...
If you are one of those who like to close your player to panel and see what it plays in your conky?
Sayonara can do that over dbus, whether you like just artist and title, a progress bar, or even the cover art in your conky.
Pro Sorts by the users options
Has everything you may need onboard.
Pro Many plugins standard
Has many plugins out of the box, e.g. internet "radio"streams, equaliser, sprectrum analyser, bookmarks, audio converter, broadcast, playlists, lyrics for the song you're playing, and more. It's the full music enjoyment experience. If you like minimalistic, Sayonara can do that too.
Cons
Con GTK App
Its a GTK app so integration in other desktops is terrible it also uses GNOMEs ClientSideDecorations so it will break many window managers.
Con No equalizer
Lollypop still does not have an equalizer.
Con Good party mode
Con No In-App Volume Control (0.9.242)
It has no in-app volume control, it has to be managed through system 'Sound control/Applications'

Con Requires a well organized music collection
Lollypop will be a pain to use if music is badly tagged. The setup is a one time thing, but it can be a pain to organize a large library.
Con Playlist driven play back
If you install Sayonara, hook it up with your music directory, and want to have it play by clicking a music file and let you surprise yourself with its shuffle... it won't. It will play that double clicked song, but then just plain stop untill you double click that next one. To have it just play through your collection, you first have to make a playlist, and only then let Sayonara play that (randomly). No deal breaker, but certainly an non-intuitive hassle preventing "click that in my library, play and don't bother me no more".
Con Dynamic playback
Sayonara works with a dynamic mode shuffle method, that quite often eliminates any surprises and comes with "the same" suggestions when your collection is smaller. It can be switched off though (bottom left switches, under your playlist) if you don't like it. All the options will require you to "look into it" though, since it is somewhat confusing...
