The short version
Pick Soundslice if you learn from recordings and videos: syncing notation to a real performance is what it does best, and it works on any device with a browser. Pick guitarnotation if you want your tabs on your own computer, offline, without an account, and paid for once.
Price
Soundslice has a free plan, and Plus costs $50 a year; PDF export is on paid plans only. guitarnotation costs $25 once, about half a year of Soundslice Plus, with every feature included and nothing to renew.
Where your songs live
Soundslice runs in the browser and needs a free account. guitarnotation is a desktop app for Linux, Windows and macOS: your songs are files on your computer, it works offline, and there's no account. Nothing you write is uploaded anywhere.
Files
Both open Guitar Pro files and MusicXML. guitarnotation also opens MIDI, which Soundslice doesn't, and exports PDF, .gp, MusicXML, MIDI and WAV. See MIDI to guitar tab.
Reading and practising
Both show tab and notation together, a fretboard during playback, and slow down and loop. Soundslice can play along with a real recording; guitarnotation plays the notes with a General MIDI SoundFont, and can export that as a WAV file to practise with anywhere.
guitarnotation's player and editor also run in your browser as a free demo, no account needed. Or see all the alternatives.