The comparison isn't that bad. You can 'soup up' a jar of stuff - let's say green curry paste rather than cantong/Kantong whatever (I've never used Kantong anything) and make a great meal. Add some extra galangal, lemongrass and/or lime leaves to the base paste, use quality chicken and vegetables, add some tamarind, whatever.
You can also (something people often ignore) totally stuff up your 'from scratch' curry paste - too much fish sauce, leave unpalatable bits of lemongrass fibre in it, fail to remove all the tamarind pips, too much/too little chilli, burn it, overdo the coconut, etc.
AG gives you more potential control but also more potential stuffing up points. Also making the paste is one thing, turning it into a good meal is another.
All things being equal, I prefer the paste to the jar but things are not always equal and paste alone won't make a good curry.