It super optimized for both retail and classic.
You can even configure the hell out of it, make it select quest rewards automatically, auto equip on stat scales, choose how convenient vs fast you wanna travel. And skies the limit, if you pay they will very often add feature requests too and are very fast at handling bug reports,
To be honest, just the auction house plugin and synced databases across realms with history is well worth the cost in itself
Which is ironically the only reason you would pay because that needs live sync trough the windows app.
AAP is amazing. IF you follow it perfectly to the T from level 1. If you start it late, or skip too much the whole thing just breaks and loses its placement. I wouldn't recommend it for a brand new player.
Questie is probably the best since it shows all available quests on your world map so if you get lost you can very quickly see where there might be a quest hub you missed. Also note that 30-40 is probably the worst part of Classic leveling and it speeds up a lot past 40.