The short version
Pick Guitar Pro 8 if you write full band arrangements with drums, need several voices on one staff, or want the most realistic playback. Pick guitarnotation if you read, practise and write guitar or bass parts, want to pay $25 instead of $69.95, or use Linux, where Guitar Pro doesn't run.
Price
Guitar Pro 8 costs $69.95. guitarnotation costs $25, paid once, with every feature included and free 1.x updates. Neither needs a subscription or an account.
Files
This is where switching usually gets stuck, so it matters most. guitarnotation opens .gp files from Guitar Pro 7 and 8, .gpx from Guitar Pro 6 and .gp5, .gp4 and .gp3 from the older versions, and exports .gp files that Guitar Pro 7 and 8 open. You can move a song back and forth, or share files with friends who use Guitar Pro.
Two things don't come across: drum tracks, and second voices on a staff. guitarnotation has one voice per track and no drums, so it leaves those out when it opens a file and lists what it skipped. Both programs also open MusicXML and MIDI, and both export PDF.
Reading and practising
Both show tab and standard notation together, both show a fretboard during playback, and both slow a passage down and loop it. guitarnotation shows the current chord's shape faintly behind the playing note on the fretboard and highlights the chord diagram you're on, which helps when the melody sits inside a chord.
Writing tab
Both let you enter notes from the keyboard. Guitar Pro has more notation: more voices, drums, lyrics and a wider set of symbols. guitarnotation covers what guitar and bass parts need, including bends, slides, hammer-ons and pull-offs, harmonics, vibrato, palm mutes, grace notes, tuplets and dynamics, for guitar, bass, 7- and 8-string, banjo, ukulele and mandolin.
Sound
Guitar Pro plays back with its own realistic sound engine. guitarnotation plays through a General MIDI SoundFont, GeneralUser GS by default or any .sf2 you like. It sounds good enough to practise with, but it isn't Guitar Pro's sound.
Linux
Guitar Pro 8 runs on Windows and macOS. guitarnotation runs on Linux as well, on X11 or Wayland, and one licence covers all of your own computers whatever they run.
Still deciding? Try the player and editor in your browser, or read about the other Guitar Pro alternatives.